>>>the free figure is now at zero
So something's hogging all of the memory.
Take another look at the 'mem usage' column. If you're certain that the only unusual behaviour is from Firefox, try resetting Firefox.
To do so, first go to Tools > Add-ons > Extensions and note down the names of any extensions that you want to keep (because you'll need to reinstall them).
Then go to Help > Troubleshooting Information and click 'Reset Firefox'. Click the button that asks you to confirm you choice.
Then (before reinstalling the extensions) see if the problem has gone away.
However, remember that it's not only a high 'mem usage' figure that can point to a problem. If a figure keeps going up and down (even though it's never actually very high) that could also indicate a problem. As an example, people who use Microsoft Security Essentials often find that 'MsMpEng.exe' (which is the program's updater) can exhibit such behaviour. (But you shouldn't even have that program at all if you're using AVG!)
Having written that, it occurs to me that AVG might actually be the cause of the problem anyway (if it's constantly trying to update but can't do so). Try temporarily disabling it to see if it makes any difference:
http://www.avg.com/ww-en/faq.num-3857