Situation: since a few months back, sometimes, when I middle click on a link in
Firefox or click in Thunderbird to open a new tab with that link, that tab is
empty.

If I go to that tab and hit enter on that URL it does nothing. If I type
another URL on that tab, it does nothing. That tab is dead. I can open another
new tab, paste the same URL, or repeat the click on Thunderbird, opening
another new tab, and then it works. Or not.

When this issue started, it would be one tab that would not work. The number of
non working tabs increases, now sometimes it is 4 tabs that do not work.

This doesn't affect other instances of Firefox, opened with a different
profile. And so far, only my big desktop computer is affected. This Firefox
instance may have dozens of windows and hundreds of tabs. Machine has 32 gigs
of ram, AMD CPU.