I too downloaded Firefox 1.0 a couple of days back and needed to download the Java runtime for quite a few sites to work correctly. Your other browser is probably IE6 this already has Microsoft's version of java and that is why you could access the sites.
Sun's version of Java, the one you have just downloaded is much better and safer.
I also found that the spell checker I use for IE6 does not work with Firefox but this one does, just downloaded it and it works fine in forums and web pages. click here
Don't forget to scroll down and select the English UK version before downloading. After the download close Firefox and reopen and use the right click of your mouse in any message box such as the one you use here, the dictionary will appear.