1 Answer

There are a few possible answers:
You cleared your history. Without remembering where you were, Firefox can't get you back. But unless you do this constantly without knowing it, this is probably not the problem, which leads us to...
You're going back to far. Firefox can only remember so much of your history, if you try to go back a bunch of times in a row, you'll only get so far. I think ten or so is the limit, but I'm not sure of the exact number. But again, unless you're trying to go really far back and you're surprised you can't go any further, this probably isn't the problem as well, so I'm betting on...
You opened a new tab. Tabs are the things that show up in the row under the Bookmarks Toolbar with the title of the page and a little icon. When you open up a new tab (by middle clicking a link, hitting Ctrl+T on a PC or Command+T on a Mac, or sometimes websites open up other pages as tabs when you click on a link). They are like opening up a new window without actually opening up a new window, and they has their own history. This means that you can't from site A to B to C, open D in a new tab, then get back to A. In this case there are two histories, A, B and C is it's own thing, site D is it's own thing. If you hit Back on site D, you can't, this is the first page in this history, you can't go back further than the beginning of something. BUT GOOD NEWS! It's really easy to fix, if you want to go back, just click the tab you were on before (in this case, the tab with site C), and you'll be able to go back through that history. If you've already closed that tab, hitting Ctrl+Shift+T (or Command+Shift+T on a Mac) will open up the last closed tab, and then you're all set.

Source:http://answers.yahoo.com/question/index?qid=20090326115128AAT8NGj