See my reply in this thread about removing the @namespace line.

• /questions/1274416#answer-1279083

olds97_lss said

I had the userChrome.css update done for 65, then 71 and now with version 72, the tab bar and bookmarks bar merge into one line. Looking for an update to fix it as the issue is beyond my abilities.

I've pootled around various Firefox help places today after My Fx updated to 72 and the Tab catastrophe arrived yet again.

Any known answer surfaced yet that works? As the few suggestions I've tried all fail to untangle the mess.

Ax

Mmnnn! Having searched around some more it does seem that removing the full ".../there.is.only.xul"); line at the top of the userChrome.css file will resolve the problem.

There was suggested (Elsewhere) another line to replace it, but not knowing much about .css I left that suggested line out and the userChrome.css still works okay.

Ax