OMG I got what I asked for half March this year and then forgot to check: you can group tabs in Firefox 💖
This is available in version 138 and I love it!

Simply click on a tab, then 'Shift-click' on the tabs you want to group with it, and right-click on one of them.
Choose 'Add Tabs To Group' in the pop-up menu.
You can choose to add the tab to an existing group, or to create a new one.
I mean 💖

Next; I want to use my own colours. Make it so.

More on this:
https://blog.mozilla.org/en/firefox/tab-groups-community/

@pascaline Hmm, I'm on 138 but I don't seem to have the feature. What's the exact version you have?