One long-standing feature-request I have since almost ten years is that, I need the tab bar to be scrollable. This is the main reason why I can't use Opera or Chrome and have to use Firefox. When working with 20+ tabs like I do often, then tabs become to small to even read anymore. See for the problem https://imgur.com/a/fdU6y In Firefox the tabs have a minimum width which at least let me read the first ~8 letters, which helps a lot! And I think it would help many others.
I don't suggest to make that mode the default mode, but at least I would like to be able to flip a switch to be able to use it. What also comes of use is this addon: https://addons.mozilla.org/en-US/firefox/addon/tab-mix-plus/ Which allows me to use 2 rows of tabs. This also is very helpful.
There don't seem to be addons for either of these addons, which makes me think, whether that the addon API doesn't support addons regarding the tab bar layout?