If I may pull some statistics from the experts at Secunia...
Firefox: Mozilla Firefox 1.x with all vendor patches installed and all vendor workarounds applied, is currently affected by one or more Secunia advisories rated Less critical (2/5)
Currently, 4 out of 17 Secunia advisories, is marked as "Unpatched" in the Secunia database.
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Opera: The Secunia database currently contains 0 Secunia advisories marked as "Unpatched", which affects Opera 8.x.
Currently, 0 out of 0 Secunia advisories, is marked as "Unpatched" in the Secunia database.
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Netscape 8: The Secunia database currently contains 0 Secunia advisories marked as "Unpatched", which affects Netscape 8.x.
Currently, 0 out of 1 Secunia advisories, is marked as "Unpatched" in the Secunia database.
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Secunia
claims that Netscape 7.2 is Less Critical (2/5), but I do not believe this claim, as there have been numerous security flaws in the underlying Mozilla 1.7.2 codebase of Netscape 7.2. Mozilla has since been upgraded, while Netscape 7.2 has not. My reccomendation: don't use it.
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My own web browser, [sdt=7971]Donzilla 0.7 PR2[/sdt], is subject to the same vulnerabilities as Mozilla 1.7.8. Both have a Less Critical rating and have three bugs.
My personal reccomendation: use common sense. I trust Donzilla and Mozilla 1.7.8 to be secure, and so far, secure they have been. It's common sense that prevents holes from being exploited. In Internet Explorer, though, it isn't. There are too many backdoors that can be exploited just by having an IE window open -- you don't even need to visit a web page. Take it from somebody that had it happen -- just days ago I believed that I could use IE on a server in the basement just to download PHP and MySQL. Having accidetnally left it open longer than I needed to, I came back and found my server a mess of viruses and spyware, so bad that my ISP called me and said to take the thing offline.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.7.8) Gecko/20050511 Donzilla/0.7PR2 (WML/1.3; WML/1.2; WML/1.1; WML/1.0)
Laptop: HP Compaq nx6325 - Turion 64 X2 @ 2GHz, 2GB DDR2, 100GB HD, ATI Radeon X300, 15" LCD, Seven Pro
Handheld: Palm Treo 650 - Intel PXA270 @ 312MHz, 10MB RAM, 32MB flash, 2.7" LCD, Palm OS 5.4