Memory Restart Tells You When Firefox’s Memory Usage Gets Too High (and Restarts It For You)

Firefox: Memory Restart tackles Firefox's well-known memory leak problem by telling you when they get out of control, and restarting Firefox automatically when they do.
Firefox still has trouble managing memory sometimes, and if you open a lot of tabs or leave it open for a long time, it can slow down to a crawl. This is especially bad if your computer doesn't have a lot of RAM. Despite this issue, many people still use Firefox for its other advantages. If that's you, you can at least keep a lid on slowdowns with Memory Restart. Memory Restart display's Firefox's memory usage in the add-on bar, and gives you the option to automatically restart Firefox when that usage gets past your comfortable level. You can also restart it manually with the click of Memory Restart's button, which you can put in the toolbar or the add-on bar.
Note that Memory Restart uses Firefox's about:memory to determine how much its using, which may be different than what you see in the Task Manager. Either way, all you need to do is keep an eye on Memory Restart's display, and note the threshold at which Firefox starts to slow down for you. Then, set that as your threshold in the preferences, and the button will turn red (or restart automatically) whenever Firefox reaches that level of memory usage.
Memory Restart is a free download, works anywhere Firefox does. If you're on Windows, you'll also want to check out previously mentioned Memory Fox, which tries to manage Firefox's memory for you.