If you go to the about:config page you can edit browser.tabs.tabMinWidth down to 50 (default is 76) which lets you fit a bit more If you want more than that it’s possible by editing the userchrome:
old.reddit.com/r/…/can_i_get_below_50_tab_width/
Modifying userchrome is admittedly difficult if you’re not a developer… but not out of reach if you’re able to follow a guide.

The blackout was fairly short, although some subreddits did continue it. But the main reason a lot of these subs stopped the blackout was reddit kind of forced their hands to reopen subs. They sent messages to all sub mods saying if any single mod wants to reopen the sub to get in touch with them and they’d basically get the head mod position and be able to take over. They also implied that if subs continue to stay private they’d find someone that is willing to take over moderation responsibilities so the sub could reopen. The solution to this that moderators came up with was to open the subs but purposely sabotage them with ridiculous rules. Preferably ones that would require the sub to become a full time NSFW sub.