Maybe one shouldn't suggest two things in one post, but for me these two belong together. I really liked how in Opera in the "good old days" you could place the tabs on the right hand side.
I know this feature exists in Vivaldi, but I love a lot of the current Opera features and don't want to change...
AND it shouldn't just be "tabs on the right side": it should be more like the feature rich (but outdated) extension "Sidewise":
-> Tab-Tree, Hibernation, Folders, Titles, Highlighting... a "sidewise for sidebar", so to say :-)
I haven't found anything like that for the "extension sidebar", which seems to me would be the best place, IF one could switch the sidebar to the right...
PS: If any reader knows of something like that, PLEASE let me know! Cheers!