Been there most likely, "Asked 15 years ago," and I am not sure xmodmap is still supported. Years ago I searched a lot, and there were many "try this," yet nothing worked, at last persistently and I went back to work on Windows. However, I vaguely remembered a promising exception from a certain Ace, thank God for him, so I searched for past posts of mine and found it, but we found it did not work past sessions.
For me it is important due to stiff arthritic typo-fingers, and as I do a lot of cut and paste, and do not want to take even more time then I often do typing replies on forums, etc. I must lift up my hand for each letter and try to land on the correct key, but about every fourth letter is wrong unless I am very slow. And I also hot key a lot of things. Thank God for AutoHotKey.
However, as described above, since working keyboards do ot work (no a or n keys) on my new Manjaro KDE Plasma (once installed), then that put a full stop to trying it out.
Maybe it will get fixed.
yeah i havent found too much on remapping- there are customizable hotkeys, but that doesn’t remap keys
It s new... Input Remapper:
https://www.linuxuprising.com/2020/12/remap-keyboard-and-mouse-buttons-on.html
And within the Settings section are a multitude of GUI customization options, which includes multiple tab rows. Review 1.
For multiple tab rows, go to about:preferences#design ans scroll down a bit and under Tab Bar Style check Multi-Row Tab Bar. You can also choose Number of rows when multi-row tabs are enabled. Above that, I prefer Firefox Proton UI.
As for themes, I have tried many, and want good visibility of text, as well as the active tab being identifiable/framed, and find two the best provide this, that of Better Darkmode by Unknownplayz, - presuming you like dark mode, which I find provides the best visibility.

As regards extensions, there is the Bible Previewer Extension, which turns references (as in 2 Pet. 3:9) to viewable pop up on mouse hover.
Then there is the Custom Scrollbar extension, by which you can customize Go to Options and choose New Scrollbar if you want, and move the slider below on right to Use Custom Colors, and choose contrasting thumb and track colors. Below that, under Advanced, I disallow web sites from overriding settings.

And an extension I often use is the Undo Close Tab Button by Button Guy, which places an icon on your extensions bar which you can click on and see and choose a tab that was closed.
Thank God.
Another helpful extension for me is the Form History Control which saves text such as on this forum (not banking sites) so as to recover this later if needed. 
Note also though that the advantage of installing multiple browsers (I have about 8) means that each can be used for its own purpose (like some for certain forums, other for certain shopping sites, etc.) For doing that with Firefox portable, see here, while for Floorp, I just install and run it from different locations. I also do run two "stand-alone" installs of Chromium-based Vivaldi, which is another portable browser.
And also as regards recovering that which was lost, is the Open in the Archive (.org ) extension, but which you can r. click on a link and choose to open it in the Internet Archive, and thereby obtain a defunct link, or compare changes. I have many more extensions - thank God for such - and other posters can add theirs, as this should be a start.