With FF IIRC there were a few releases around 15 (?) that weren't up to snuff. I think I even had the browser crash a couple of times, which is unusual for me. If that's when you switched to Opera I'm not surprised.

>So, with 10+ tabs open all showing charts from Google Finance Opera crashes out, and then can't restart properly. Have uninstalled, reinstalled, tried 32 and 64 bit versions, several times - no luck once it crashes. So went back to Firefox and after the most recent upgrades to FF and Adobe Flash the whole thing seems very stable again, speed stays fast and memory usage stays constant (which was the reason I looked at alternative).
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>>So, I recently dropped FireFox as my main browser and switched to Opera. I didn't use Opera previously because it was a pay-for-browser (or you had to endure banner ads etc.) but now it is free like the others.
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>>The main reason I switched is because (1) FF is 32 bit (although they have a 64 bit in beta apparently but it reverts to the 32 bit version on upgrades), and (2) it became very sluggish when running on my Windows 7 64 bit computer with multiple tabs. The memory usage went to +/-1 Gb and everything just became crap - poor response, mouse getting stuck, etc. I would then have to close FF and restart it and it would be OK for a while again. And then the cycle repeated itself.
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>>I installed Opera 64 bit and it runs like a charm. For the same tabs I had in FF the memory usage has now run up to about 2.1 GB and has leveled out at that over the past few days but the response of the browser and computer in general is perfect. I'm impressed.