Upgraded to Firefox 89 this morning and I have to say that I do not like the new appearance! Within 10 minutes I’d found a few articles about how to customise and it make it look a bit like the previous version.
For comparison, this is Firefox 88.
And this is Firefox 89.
A very helpful article over at https://www.ghacks.net/2021/05/09/how-to-fix-the-firefox-89-user-interface/ linked to a GitHub repository https://github.com/black7375/Firefox-UI-Fix and after installing this customisation, the latest version doesn’t seem so bad.
This is Firefox 89 after customisation.
It’s not the same as 88 but at least the individual inactive tabs are a little more obvious and I can always edit the styles for a little extra contrast between the active and inactive tabs if I want to.