It looks like you will need to update your userChrome.css theme modifications that put the tabs on the bottom. Alternatively you can disable them by going to about:config and changing toolkit.legacyUserProfileCustomizations.stylesheets to false then restart.

Hi,

I prefer the tabs on the bottom, what would I need to change/add to userChrome.css to allow this have the + tab stay?

Thanks

You need to remove this CSS rule in 108+

#tabbrowser-tabs { width: 100vw !important; }