I just don't get why this would be useful. When I start Firefox, inactive tabs aren't loaded until I click on them. If something is eating RAM, I can close that tab or close Firefox, and very rarely Firefox would crash. Why would someone like me, that doesn't suspend/hybernate their computer, want this and get a tab unloaded without my explicit will to do so?
The feature stops (or reduces the likelyhood) of that happening. It literally is made to trigger in a situation where the alternative is to crash.
>So, if I want to disable this, I should set a big number to the browser.low_commit_space_threshold_mb pref in about:config, right?
This will ensure that you crash and have to reload all of your tabs instead of the oldest ones, yes :-)
When I start Firefox, most tabs (4 or 5 nowadays) won't load, but be there as placeholders for things I need/want to solve through the week (e.g., Amazon search for a gift for my nephew, Youtube list of tutorials). They won't load until I'm ready to use those tabs.
On the other hand, I've had tabs loaded from the start of one session and used sparsely through the day. I would not want those tabs to be unloaded without my consent, even if they are memory hogs, as it would interrupt my workflow. Crashing is fine (and accepted as a last moment failsafe move) by me.