No. Type "about:config" into the address bar (without the quotes) and hit Enter. Then scroll down to the entry for "browser.sessionhistory.max_total_viewers" Double click on the -1 and change the value to zero. Hope that works for you.
Yep, that portion of Firefox is in English, it doesn't matter if I am using the German release. Thanks for the hint.
as far as Firefox taking more memory, it also seems that Firefox stores the uncompressed images in memory as opposed to storing them as their original format (jpeg, gif, bmp, whatever). There was a linux user who did some experimenting and found that by changing a few lines of code or something that he cut FF memory usage in half just by changing the way it stores images it downloads on loaded pages. It was a fairly interesting read that I saw on digg quite awhile back.