It sounds like Tree-style Tabs will stop working then as well. That will be a sad day. Tree-style Tabs and some form of mouse gestures are pretty much all I need.
I won't need either of those, but this is a major part of why I like Firefox, each person can make it their own.
These in particular make Firefox my own (your Firefox is not my firefox, but your firefox is OK), and I will be sad if they disappear without replacement:
The only plugin I really care about is ublock origin. If that stays, I'll keep ff otherwise I'll go back to chrome. I suppose there's no way of knowing whether a given plug in is possible in future without having access to the source.
most of those will end up with webextension versions. I don't know what they all do, so i can't say all.
I do know that ones like ghostery and lastpass have financial incentives, while others (like vimfx) are already planning on it. Others don't seem to rely on many (if at all) XUL/XPCOM features
It's just incredible that they're going to do away with XUL. Imagine if Google somehow decided to remove all the Android apps in the Play Store and told everybody to rewrite their apps or die. And what's even worse, most addon writers don't even receive money for that.
This is going to kill the addon community but I guess they don't care at all.
These in particular make Firefox my own (your Firefox is not my firefox, but your firefox is OK), and I will be sad if they disappear without replacement:
Disable Style Button https://addons.mozilla.org/en-US/firefox/addon/disable-style...
Disconnect https://addons.mozilla.org/en-US/firefox/addon/disconnect/
Flash Control https://addons.mozilla.org/en-US/firefox/addon/flash-control...
Ghostery https://addons.mozilla.org/en-US/firefox/addon/ghostery/
It's All Text https://addons.mozilla.org/en-US/firefox/addon/its-all-text/
LastPass https://addons.mozilla.org/en-US/firefox/addon/lastpass-pass...
One Tab https://addons.mozilla.org/en-US/firefox/addon/onetab/
Pinboard https://addons.mozilla.org/en-US/firefox/addon/pinboard-exte...
Privacy Badger https://addons.mozilla.org/en-US/firefox/addon/privacy-badge...
Suspend Tab https://addons.mozilla.org/en-US/firefox/addon/suspend-tab/
uBlock Origin https://addons.mozilla.org/en-US/firefox/addon/ublock-origin...
Uppity https://addons.mozilla.org/en-US/firefox/addon/uppity/
VimFX https://addons.mozilla.org/en-US/firefox/addon/vimfx/
I have a few others which aren't much important to me, but these make FF my own.