Really? I removed my tab bar years ago (I use Tree Style Tabs) and never had to tweak my userChrome.css once.

I remember it breaking exactly once. Last year they changed the syntax. But before that it was running fine for 3 or so.

Oh no! I am on ESR, so I guess I haven't gotten this change yet. How did you fix it?

Here is the change (as reflected in the sidebery wiki):

https://github.com/mbnuqw/sidebery/wiki/Firefox-Styles-Snipp...

It was Firefox version 113 that changed it.

> I really wish Firefox would make the UI more stable and make user chrome mods some kind of plugin system where I can pick and choose how to deeply customize the interface.

Firefox extensions before Mozilla decided to kill XUL had the capability to radically alter the UI. This is what made FF extensions so powerful.

There were arguably good reasons for them to remove XUL[1], but it created a void of functionality that previously existed. I still think that a more secure version of XUL could offer a better user experience, but unfortunately we're forced to settle on the severely limited WebExtensions standard nobody is happy with except browser developers.

[1]: https://yoric.github.io/post/why-did-mozilla-remove-xul-addo...

> I still think that a more secure version of XUL could offer a better user experience

In a way, webextension experiments are exactly that; you have complete access to all the “privileged” APIs in the browser, just like XUL extensions did. Problem is you need to toggle some prefs and use Dev or Nightly builds.

When they removed XUL, Mozilla promised to provide relevant APIs to support at least some top extensions.

Obviously that never happens, And TabMixPlus (the top1 extension) died immediately.

Used to do this but then I switched to Floorp which is a fork of Firefox with this built in, + some other cool customization options

> Unfortunately my css seems to break with every other Firefox release. It almost feels like it's being done intentionally. I recall how after one of the updates the entire contents of my custom userchrome file was automatically bracketed with css equivalent of `if false then`. It could simply be fixed by setting read-only attribute for the file but after some time they started randomly renaming css features without any obvious reason. It's like they are trying to make you to give up on customization and force to use the crappy default ui.