Thinkpad x200s (2009) with Core2Duo SL9400, 2gb RAM, 128gb SSD + AntiX Linux with herbsluftwm.
Firefox will not perform smooth here, but very usable. I watch youtube video from Yewtube on Terminal + mpv, browsing only on Lemmy, news on Newsboat RSS feed, Palemoon if some news sources are heavy, and download with wget.
IMO if you experiencing problem on Firefox with that spec, I think there's a problem with your Operation System or your OS is very hungry power. I recommended using Q4OS linux with pure profile installation for minimalist system out of the box in KDE environment. The performance are very good on my Thinkpad x260..