If you visit two or more particular websites every time you start Firefox, you can set both of them as your homepages. Setting "[Site 1 URL]|[Site 2 URL]" as your homepage in Firefox will cause both sites to load the moment Firefox is started.
JGZ also mentions the use of bookmark folders, which is an excellent way of storing lots of bookmarks within that little bar and/or accessing several similar sites at once. I use this to great effect in Safari.
Firefox is also semi-intelligent, like Chrome. If you visit one particular website enough times, all you need to do is type a portion of that site's title or URL into the address bar, and the site you want will show up as the first autocomplete result. As an example, I've visited HLP so many times that simply typing in "hard" or even just "h" into the address bar gives me a link to the list of Updated Topics as the first result.