Hi, we won't keep backward compatibility forever. You have to replace your css file with that from https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome . His version works with 131.

It is too bad this is made so complicated. I cant tell that FF is loading any css file that I point to in this version. I tried just renaming MrOtherGuys multi-row_tabs.css to usrChrome.css in the chrome directory of my profile. Restarting FF shows no change.

I also tried using @import url ("multi-row_tabs.css"); in the usrChrome.css file to indirectly load the css. Nada.

MrOtherGuy is sloppy in the examples. He uses the @import url("file") and @import url(file). Im far from expert, but either the quotes are needed or they are not I would think.

The flags : userchrome.multirowtabs.full-width-tabs.enabled

userchrome.multirowtabs.scrollbar-handle.enabled

are set true but seem to have no effect on the width of tabs.

I wish there was an easy way to test if FF was loading anything css related.

In fact I find on the about:support page that Legacy User Stylesheets is active but no stylesheets are found. I am reasonably certain that I am using the correct profile directory as I can get there from the open_folder button an the same page.

I tried just renaming MrOtherGuys multi-row_tabs.css to usrChrome.css in the chrome directory of my profile.

It should be userChrome.css, not usrChrome.css.

Ok. Persistence and better glasses has shown me the error in my method. I seemed to have lost that little "e" that makes the difference between "usr" and "user".