I presume you mean Mozilla Firebird rather than Mozilla proper?
If it is Firebird, then just right-click where the icons should be and choose the 'Customise...' option. Then from the menu which appears, just drag the forwards and backwards icons onto the toolbar. Hopefully that should work.
Have you recently installed a new skin? You might want to try switchng back if so (Tools > Options > Themes). If the worst comes to the worst you can always just delete your current profile and create a new one.
If it's been installed for a while try changing between a few themes, if that doesn't solve it I think you'll have to reinstall. I've had the same thing happen a few times in the past and it's always been solved by installing the latest nightly. FB seems to get slightly weird once it's been used for a while, especially if you're using the nightlies to start with.
If you need to delete your profile go into, say, "C:\Documents and Settings\Username\Application Data\Phoenix\" and delete the subfolder. Alternatively, if you alter your Firebird shortcut and add "-p" to the end of the command line, you'll be able to create and delete profiles using Firebird's profile manager, which may well be a nicer way to do things.
N.B. That deleting your profile will remove your bookmarks and usually any extentions your are using, together with the more obvious things like your settings etc. Be sure to backup anything you need first
P.S. The next milestone release, 0.8, will be out on Monday