I think it's the specific pages - some Javascript is much more memory hungry that others.
A few hundred HN or Wikipedia tabs work absolutely fine, I've tested. But there are much heavier sites, and only a few of those can cause the machine to start swapping until OOM, especially when left to work in the background.
(And unlike Chrome, Firefox does not seem to throttle/unload background tabs automatically... or if it does, it does it much less aggressively)
A few hundred HN or Wikipedia tabs work absolutely fine, I've tested. But there are much heavier sites, and only a few of those can cause the machine to start swapping until OOM, especially when left to work in the background.
(And unlike Chrome, Firefox does not seem to throttle/unload background tabs automatically... or if it does, it does it much less aggressively)