Some things in addition to the above can also consume lots of RAM:
If you are not using an ad blocker, some code used by some ad companies are so memory intensive that they eat lots of RAM rather quickly. The easiest fix for those is to run an ad blocker. (I personally use uBlock Origin with mostly just the default settings.)
Leaving Firefox open for several days can eventually eat a good chunk of RAM. If practical, it is suggested to close and restart Firefox every day. (I start up Firefox at the start of my day and shut it down at the end of the day.)
If the GPU driver isn't compatible with Firefox, "hardware acceleration" could both slow down Firefox (from crashing the thread trying to use the GPU) and end up grabbing a lot of memory in the process. The suggested fixes are, first, try to get the most recent supported graphics driver from the graphics chip manufacturer, and if that doesn't take care of it, turn off "hardware acceleration" (in Menu -> Settings -> "General" page -> uncheck "Use recommended performance settings" and then you can see and uncheck "Use hardware acceleration when available").
Some things in addition to the above can also consume lots of RAM:
If you are not using an ad blocker, some code used by some ad companies are so memory intensive that they eat lots of RAM rather quickly. The easiest fix for those is to run an ad blocker. (I personally use uBlock Origin with mostly just the default settings.)
Leaving Firefox open for several days can eventually eat a good chunk of RAM. If practical, it is suggested to close and restart Firefox every day. (I start up Firefox at the start of my day and shut it down at the end of the day.)
If the GPU driver isn't compatible with Firefox, "hardware acceleration" could both slow down Firefox (from crashing the thread trying to use the GPU) and end up grabbing a lot of memory in the process. The suggested fixes are, first, try to get the most recent supported graphics driver from the graphics chip manufacturer, and if that doesn't take care of it, turn off "hardware acceleration" (in Menu -> Settings -> "General" page -> uncheck "Use recommended performance settings" and then you can see and uncheck "Use hardware acceleration when available").