Firemin (the new name for Firefox Plumber) is a tiny tool that's been designed to minimise Firefox's RAM use.
The program couldn't be any simpler to use. Download it, unzip the file, run foxBoost.exe - and that's it. No installation required, no settings to configure, Firemin simply runs in the background, looking for the Firefox.exe process and, if it's there, uses Windows API calls to reduce the browser's memory consumption.
The program works, too. Well, sort-of. On our test PC, after launch Firefox required around 120MB of RAM. We ran Firefox Plumber and this dropped to under 1MB. And stayed that way, most of the time, regardless of how many tabs we opened.
So what's the catch? The first problem is that Firefox was storing as much data as before, it's just now being swapped out to the Windows paging file. We checked this by using Task Manager to monitor page faults, the number of times data had to be fetched from the paging file because it wasn't in your physical RAM: after completing a test under normal circumstances it was 128,000, but with Firemin running it reached more than 10,000,000. So your PC is having to do much more work to maintain these low RAM levels.
And there's a sign of that in Firemin's own CPU usage. On our test system it was permanently using 3 or 4% of our CPU time, and in total needed even more attention than the System process (the Windows kernel).
For all this, our system wasn't noticeably slower. And so if you have a computer that's short on RAM, and you always have Firefox running, and you've noticed that it gobbles up more memory than you'd like, then Firefox Plumber may be useful - give it a try, see if the program works for you.