It's unbelievable how badly some simple-looking sites are designed. If you're saying that with just a few 'light' websites open, and not doing anything in Firefox, that Firefox just starts taking up more and more memory, then it's plausible it's just Flash or some script going nuts. You can disable all add-ons by starting Firefox in safe-mode from the terminal, firefox -safe-mode, to see if you have the same thing happen. It might be Firefox, but often it's an add-on or combination of add-ons causing the leak. Your system will likely become sluggish from the other programs demanding unavailable memory. You can see that some Firefox process is causing the issue but that’s no way.

Usually, each Firefox tab is displayed as Web Content. If you use top or some other system resource monitoring tool like Glances, you’ll notice that these tools cannot distinguish the opened tabs or extensions. If Firefox is still showing massive memory usage, then it's a leak. Suppose your system is taking too much of RAM or CPU.

If you have another browser installed, like Chrome, start loading massive pages in it. Then start loading other memory intensive programs. You can check this by closing all your tabs except say a blank one. If it's a memory leak on the other hand, that would be a big problem. As long as the program is good about letting go of memory it doesn't absolutely require to other programs, it's not a problem. By keeping as much as you can in memory, it saves having to go to disk and also reloading content needlessly. Try 15000 if you have between 512MB and 1GB ram. It’s set to 50000, but you can lower it, depending on how much memory you have.

Again, go to about:config, filter browser.cache and select. It's like keeping a warehouse mostly empty because you think it's cleaner. If Firefox takes up too much memory on your computer, you can limit the amount of RAM it is allowed to us.

There's a common misconception that utilizing a lot of memory is bad. And one of the best ways to reduce Firefox high memory usage or fix Firefox using too much memory issue is by reinstalling the browser as it will clear all. Without specifying what you're doing exactly, it's hard to say this is an actual problem.

Firefox taking too much memory