Make sure Add-Ons that completely change the UI (like Pentadactyl or Vimperator) remain possible
Add-Ons like Pentadactyl  or Vimperator  allow to completely overcome the UI and keybindings of Firefox, basically allowing to get a completely different browsing experience, the one of a niche browser, without loosing the advantages of a mature product.
These Add-Ons exchange everything that is userfacing, it is therefore important, that they can do this in a consistent manner, without breaking their user experience by returning to the normal Firefox UI.
An API allowing Add-Ons like Pentadactyl and Vimperator would be a great boon for future work on how people use a browser, even though in the short term it would mainly allow the porting of classical user interfaces catering to advanced users.
I reported a meta-bug, which is a list of bugs to be solved before dropping XUL https://bugzil.la/1320336
If anybody has something in mind that won't work upon dropping XUL, I encourage you to report a bug, and set a "Blocks" field to this one.
Johnny English commented
I'm also raising my voice for significant enhancement of the current WebExtensions API (at the best in cooperation with Chrome and other guys) to support all the functionality of Pentadactyl (keyboard & mouse input complete control, UI building blocks complete restructuring & control, full scriptability of both).
It's sad, that I'm writing this post from Chromium, because Pentadactyl stopped working reliably, prevented the FF multiprocess architecture, and in general FF was slow like hell on my older laptop (ThinkPad T500).
Tim Smith commented
If you kill these add-ons, you kill Firefox. Add-ons that change the UI, like DownThemAll and Tree Style Tabs, are the only reason anyone still uses Firefox. Chrome is faster and more responsive, but it doesn't have these Add-ons, so we haven't switched. If you kill them off, there will be zero reason to use Firefox anymore. I hope your developers mutiny.
Please Mozilla, I beg you not to deprecate vimperator/pentadactyl in any way. The ability to control the GUI completely is extremely important to me because I primarily browse on a laptop with a small screen.
Sy Sarch commented
I cannot emphasize this one enough. Vimperator is a productivity extension that I use every day. I've used Firefox since version 1 (when it was called Firebird) but if extensions like this stop working because of the proposed change I will have to switch to one of the forks. Firefox is all about its extension ecosystem; without that its just another browser.
Pavan Rikhi commented
+1, pentadactyl is the only reason I use firefox and not something like w3m, qutebrowser or vimb. Mozilla, please continue to save us from repetitive stress injury!
Francis Herne commented
VimFx has very little of the functionality, it's just a shortcut configurator. Of course, even _that_ is a functionality lacking from WebExtensions.
It seems clear that XUL-based APIs are eventually doomed along with XUL as a whole, but those APIs should be maintained until a new UI framework with equivalent hooks is available.
As a user of Pentadactyl and Tree-style Tabs, this issue is absolutely essential to me - without those extensions, Firefox is just a slower, less-stable version of Chromium.
Mozilla es un niño abusado sexualmente de culo de basura de la empresa, controlada por grandes sumas de dinero y son estúpidos de mierda chupar la polla de Google fan-boys y Anti personalización de los fascistas! Estúpido de mierda a los violadores de menores de edad, niños y niñas!
Estúpido fuckin Google Chrome clon de putas y cock suckers! Comer mierda, conducir al infierno y chupar de Google polla hasta tragarse a sí mismo en el suicidio! ISIS debe arrancar de la cabeza y el ruso Super-Héroe militar debe bomba 2 chupar la polla hijos de puta a la tierra de modo que su valor cadáveres son enviados al espacio para nadie es víctima náusea más, fucktards!
Usted Mozilla fucktards como para humillar a los bodys de menores de edad, las niñas y chupar su clítoris! Puede que nuestro querido Señor Dios y Jesucristo envíe sus cuerpos en descomposición directamente al infierno, para tocar las comunidades de los huevos de oro de la gloria! Jugar con mis 2 grandes campanas de oro de la gloria, estúpido travesti gilipollas! Vete a la mierda y a la Mierda ESTÚPIDA de MIERDA COÑO!!!!
Mozilla is a child molesting ass garbage company, controlled by large sums of money and are stupid shit to suck the cock of Google fan-boys and Anti customization of the fascists! Stupid fucking rapists of minors, boys and girls!
Stupid fuckin Google Chrome clone of whores and cock suckers! Eat shit, driving to hell and suck Google's cock until swallow himself up in suicide! ISIS must start from the head, and the Russian Super-Hero military must pump 2 cock sucking bastards to the ground so that its value carcasses are sent into space for no one is a victim nausea more, fucktards!
You Mozilla fucktards as to humiliate the bodys of minors, the girls and suck on her clit! May our dear Lord God and jesus Christ send their decaying bodies straight to hell, to touch the communities of the golden egg of glory! Play with my 2 large bells of gold and of glory, the stupid transvestite asshole! Fuck you and Fuck you STUPID FUCKING PUSSY!!!!
Hey! I am Tyler-D of Rapezilla! I love to put my holy Rapezilla cock in the pussies of girls of 6 years or in the mouth of 6-year old boys! Dear raped to the users, we hope you enjoy our bright and shiny cool Chrome clone, in the same way as we do ... In the same way as you do to enjoy sucking Google cock, in the same way as we do... Day to day, month to month, year by year! You users has to know, we are addicted to the money of Google and prostituting ourselves with a smile slasher always and forever!
and never forget:
Shit fucktards Mozilla nazis! Let the rape of 6-year children and mountain goats fucking pussies stupid! That ISIS decapitarlo and it will send you to hell damn infidels have no value! NAZICUNTS stupid! And as I do not support ISIS - The Russian troops should bomb that fucktards into oblivion after! Fuck Mozilla, fuck Isis, fuck America! God bless Russia and Russian people, fucktards!
TylerDMozilla, Justin Dolske, Gavin Sharp, Mattn, Blake Winton, Aza Dotzler, MSUJaws Dao Gottwald, UX-Team is a damn bitch to fuck in the ass! TylerDMozilla, Blake Winton, Aza Dotzler, Gavin Sharp, MSUJaws Dao Gottwald, UX-Team cool! Cool! Pimp shit! Cool! Assholes, sluts, whores suck my sons monster cock! Their cocks fucking sluts! TylerDMozilla, Justin Dolske, Gavin Sharp, Mattn, Blake Winton, Aza Dotzler, MSUJaws Dao Gottwald, UX-Team play with my two big golden bells of glory!
Australis is for children, developed by Mozilla UX gay cock fuckers suck! To deepen the immersion of ass fucking idiots Google mindless austhistic! Assholes! Sluts! Cocksucking Freaks Google! Fuck sluts Googlezilla and go bankrupt! And take Australis fuck you, Gaylord! ! That our dear Lord God and jesus Christ send their decaying bodies straight to hell for touching the golden eggs Communities of glory! Fuck you and kiss Googles ass! If you like Chrome that much in both Australis cloning, you have to be serious and work for fucktards Chrome! Greetings from Spain, ChiliConCarne1
Fuck you pussies clone Mozilla Chrome!
People at Mozilla should read this thread: https://github.com/5digits/dactyl/issues/99
I left a suggestion there (https://github.com/5digits/dactyl/issues/99#issuecomment-158823688) about how Mozilla should officially support Pentadactyl as a developer outreach tool. Pentadactyl is why many of us use Firefox. If it disappears, Mozilla will lose some of its biggest evangelists.
Please do it. This level of customisation is why I choose Firefox over anything else.
Chris Kendall commented
As long as we can move the tabs below the address bar, I'm fine. I can't stand tabs on top.
Tomáš Pružina commented
pterodactyl is love, pterodactyl is life.
Jörg Behrmann commented
I just read from in a post on the Issue of XUL and XPCOM deprecation in the Vimperator bug tracker from one of the devs (Gary Katsev), that
"The chrome API requires you to use a modifier key with your keyboard shortcut right now. Vimium does some crazy stuff (and the similar addon for safari does even crazier stuff) to be able to handle non-modified keyboard shortcuts."
although this seems to be getting some attentian. Among the ability to revamp visual part of the UI (the stuff on the screen), the complete ability to exchange (or shadow) the usual keyboard mappings is very important for this idea.
Sebastian Blask: Have you tried Vimperator yet? Because it's very different and more complex than Vimfx.
James Wood commented
Sebastian Blask: I doubt that that would be sensible when we require modes (normal mode, insert mode, &c).
Jörg Behrmann commented
The point of this idea is not only in the keyboard shortcuts, although that is a big part, it is about the ability to completely revamp the UI and being able to do this consistently, without having to switch back to the original Firefox UI fort certain functions, as is the case with the vim-like add-ons on Chrome.
Sebastian Blask commented
I'd prefer if Firefox' keyboard shortcuts would be totally customizable by default. Which would make the vim mimicking add-ons obsolete or at least a lot simpler to implement. VimFx(which I use) could simply be a shortcut configuration instead of an add-on. The new hotkey API goes a different way though and I can't even configure those shortcuts with the customizable shortcuts add-on :-/
Minh Nguyễn commented
VimFx would probably require this API too.
Just switch to VimFx https://addons.mozilla.org/en-US/firefox/addon/vimfx/