Firefox, AdBlock +, NoScript, IE Tab and Download Status Bar.
The combination can't really be beaten, both from the usability and the security point of view. Nowadays I simply can't return to IE and, on occasions when I am forced to use it, I find some of it features rather awkward and/or underdeveloped. I did try SeaMonkey recently, thinking that it may be easier to have e-mail and browsing capabilities in one application, but found it to be a little more unfriendly to the user when compared with Firefox/Thunderbird. Its appearance also was almost a clone of one of the old Netscape browsers - call me eye-candy obsessed, but when compared with the more attractive GUIs nowadays it feels a little ugly.
Before Firefox though, I was an IE 5/IE 6 user, although this mainly was out my ignorance of anything better. Quite a few years back I tried Opera (early thousands) but I disliked the banner ads which displayed at the top of the screen, so quickly returned to IE.
Even earlier (mid to late 90s) I used Netscape. Although I was loyal to the earlier versions (v2, v3 and v4), the early versions of Netscape 6 and their crash prone nature drove me to IE (at the time v5.5).
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