It sounds very complicated and - of course - I absolutely do not have any idea of anything, but summing I assume it just will be a mix of (known) issues (or a single one at least, the focus issue) and more intensively that sluggishness of my system causing inconvenient behavoir of the browser window(s) / tab(s).

Alright, so I just will keep on trying some different settings / codes to find the best solution. And I will have a nearer look at such a Google account.

Hmmm, yes, indeed, all of those Firefoxes I used (and use) somehow were slow and their use of RAM was / is not low (with add ons).

Many thanks.