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Why does Mozilla keep breaking UserCSS?

I'm very sorry if it comes off as hostile. It's actually kind of out of fear that one day it will all break down.

As person on autistic spectrum I really need ability to maintain pleasant-to-eyes user layout. Even before the Quantum updates Firefox moved from pretty much perfect-looking browser to worse and worse (kinda Chrome-looking) one. That's why UserCss was a godsend.

But every few updates stuff breaks and I have to download yet another updated version of custom CSS, and often at least one option I am using actively is no longer available because firefox update broke it for good. Today was such day - after Firefox updated, suddenly tabs were way too high, extending into favorites tabs. After updating and reconfiguring (since config files are different with each iteration), four more customization options gone in between last and current CSS update.

I understand going forward and such, but if the visuals peaked in 2007, why not keep all the CSS working properly when doing so? Just do not remove old calls and such?

All Replies (1)

Here is one big change for userChrome.css and userContent.css code in Firefox 96:

In the early Proton days, we wrote styles with a switch so that it would only apply to the new design. However, that switch was removed, meaning your Proton code is now being ignored. To fix this, find sections like this:

@media (-moz-proton) {
  /* various code */
  /* various code */
  /* various code */

And take off the top and bottom lines -- @media (-moz-proton) { and the closing } -- so it looks like this:

  /* various code */
  /* various code */
  /* various code */

If you can't find the problem in your rules, you could share them using a site like (you don't need to create an account, but don't lose the link to your paste that you'll need to share it).


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