I thought I solved the memory leak problem by following tricks I detailed here in this post.
Last night when I was comparing Goowy and eyeOS the memory useage peaked out of control until I have Goowy's tab closed.
Again this morning I have only netvibes and gmail tabs and memory still went up noticably. I therefore closed netvibes and surprisingly the memory usage came down without crashing FF!
netvibes could be the culprit. I now switched to Alesti, a pretty effective but not as nice looking reader. Well I still have to watch it for a while as it looks like that all ajax driven applications don't work too well with Firefox.
By the way the OPML file generated by Netvibes can't be imported to Alesti so it would be a tedious job if you have lots of subscriptions. I have written to helpdesk of Alesti on this.