I have spent days looking over the boards for this problem. Firefox is gulping up all the RAM on every single Mac I use it on. I have a MacBook Pro at home with 4GB RAM and FF 26.0. It takes a couple minutes to build up - but it maxes out the entire 4GB and the computer slows to a halt. It is doing this on every Mac at the college I work at - I don't know which version FF. But I was in the computer lab and moved from computer to computer and everyone had all 4GB RAM swallowed up by FF. My office computer is a MacBook Pro with 8GB RAM, There it ran OK - but still used 4GB. I have been monitoring the problem with Activity Monitor. So, either the new FF needs 4GB RAM to operate or something is wrong. I ran all the diagnostics suggested in Mozilla support pages - Turned on "safe" mode, disabled and updated plug-ins, etc, etc and nothing has helped. One thread attributed the problem to Ghostery add-on - I went down that path for a while but got frustrated. I just exported my bookmarks to Safari - which makes me sad. I love Mozilla and FF but it obviously has some huge issue with Macs.