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.
Also give us what CTR does. Seriously. Do it.
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.
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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/