The only way to free up the memory is to close firefox. Closing tabs doesn't do anything. I can 'killall firefox' and then reopen it to the same 50 tabs, and the memory usage after they're loaded again will be several GB lower.

I don't know if you count memory exhaustion as a 'crash' per se.

Does opening another program cause system instability? I don't know how you are monitoring it, but it sounds like Firefox is making use of the available ram for a cache. There's no real difference between cache and free memory, so using more is only a good thing.

Yes, it does. Monitoring is easy with htop - just look at the ram and swap usage. Leave FF open too long with too many tabs, and even the swap starts to fill up and the whole computer gets laggy.

This is a well-known problem. What's hard to believe?