Firefox consume more VM ...

Today im confuse about my laptop speed , My firefox suddenly freeze but the other application still running well, i know the specification of my box is not so good but its not so bad either. I have Toshiba Tecra 8100 with Intel Pentium III as a processor and 256 MB Memory with 1 Gigs Swap Partition, When I see the system monitor at my Ubuntu , i found that firefox consume more than 100 MB, so i decide to get another browser (such as Opera), then i watch the system monitor facility in Ubuntu, then i found that firefox consume more VM than opera , please see the picture



1st position is Openoffice Writer , then firefox take the 2nd place, see Opera ? firefox has consume a double VM than Opera .(just for note, i open a three tab window in each others) . Once again, its not about the browser speed but it is more about Virtual memory Consuming