I realize the title makes this post seem more important than it really is, and Im sorry for that. I was just reading an article ( okay, actually I just remembered an article I read ) by CNET or some other fairly large news source that mentioned " ... Firefox 1.5 users being unable to visit certain web pages, with no consistencies between the unviewable pages... "
If you happen to be experiancing this problem ( I experianced it too for about 5 pages until it clicked ), and you use Firefox extensions, you might want to disable them sequentially to determine whether or not that may be the problem. FasterFox, a common and VERY popular extension, was keeping me from visiting web pages ( first thing I disabled, immedietly fixed the problem ). Considering it "enhances" network performance ( read: jerks with your network settings ), its really no surprise that it causes problems like this. Especially considering I was using the newest just-released version of not only Firefox but Fasterfox also. I saw improvement in not only load times for Firefox but browsing ( well, atleast page loading speed ), making this an unfortunet loss until the kinks get worked out.
If youre having issues, consider your extensions and whether or may not they may be causing it ( especially if theyre less than official ). For example, if for some reason you can no longer download things ( or something akin to that ), and you have a download manager, suspect the download manager may be causing the problem. Disable it, or try another similar program and determine whether or not that helps.

Hopefully, this helps.

Now, its 3:30am people, go to bed....

It's probably your settings that you could have changed in Firefox or you have been infected with a virus. My Firefox 1.5 with FasterFox works perfectly... No browsing problems, unviewable pages or download error or whatsoever.. It's still working perfectly alright.. The only thing I'm now experiencing is faster browsing speeds with FasterFox..

It jerks the network settings? No, it tweaked only Firefox configurations so enable full speed for every tab or window to load. It does not, or rather never touches your computer's network settings. Once you uninstalled Firefox, the fasterfox tweaks will also be removed because it stays together with Firefox only.

If you are trying to let us know that what the information you provided is true, include the source. Firefox users will be glad to know if this is true..

Firefox has some serious issues with processor loading. When I run Adobe Photoshop CS2 9.0 and firefox at the same time - my cpu loads to 99% and you can't do anything in photoshop - just freezes from time to time. I tried Opera Internet Explorer and many other browsers - nothing like that, just with firefox. Other than that - everything is done pretty good in firefox, no other cons.