Once i did not use any browser except Opera (for my daily life)… Since FireFox 1.0 beat or something i was using FireFox… Some comments regarding FireFox::
1. FireFox is a nice Browser..
2. But its resource hungry (due to its mass functions)
3. Book mark manager is nice.
4. Download manager is not good.
5. Skining effects after restarting Browser
6. Supports all standard formatting (MSIE =~ FireFox)
….
But my PC has only 256 RAM so i am forced to give up FIreFox and now using Opera 8.0
About Opera:
1. Opera is less RH than FF
2. Reliable
3. Fastest
4. Proven
5. Mature
6. 100% tab browsing
7. Nice Skining
8. not well HTML CSS formatting supported.
9. Supports Build in POP3 and IMAP mail client
10. RSS ATOM reader is addon
…. 
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But Opera’s JS engine is not same as FF. It doesnt support many JS objects as in FF. FF is purely Gecko compliant.
–Hasin