Your bookmarks and history are stored in your Firefox profile folder. To back up that information, see the part of these articles about saving that information for later restoration:

Backing up your information (entire profile folder)

Recovering important data from an old profile

Then if you don't have your information after you reinstall, you should be able to restore it.

I do not understand your answer. I Think you have misunderstood the question, As I said I think I have lost the firefox browser completely, It does not appear when I clicked the Firefox icon. I asked that if I re-install Firefox 4 will I lose my bookmarks, history etc? How it all started is that I was upgrading to Firefox 4 and when I ran it, it kept freezing, showing "Not Responding" messages, so I restored it to before I upgraded and now I cannot get it at all.

I can't predict whether you will lose bookmarks and history when reinstalling Firefox.

So I recommend you make a copy of your profile folder so you have a backup in case that folder is damaged or wiped. Normally it's fine, but better safe than sorry.

If you start up Firefox (successfully) and your bookmarks, etc., are missing, then the second article above describes how to restore them.

So, in brief, back up then reinstall.

I don't understand what you mean my profile folder, where would I find this? how do I make a copy and on to what? CD on or the C drive in my computer?

Your Firefox profile folder is in your c:\users\username\appdata somewhere. The articles linked above have tips for finding it.

You can make a copy somewhere else on your C drive. It's just in case something goes wrong with the automatic conversion. Note that your profile folder contains your history and saved passwords, so for privacy reasons, you may want to delete the backup when you don't need it any more.

I seem to be getting nowhere with this, I cannot find c:\users\username\appdata on my C drive. I opened up "Backing up your information (entire profile folder)" above and followed the instruction then it asked me to open Finder, I can't find this anywhere, where is this?

I didn't come back to you because I have been away the last few days

Sorry the original post says Windows 7, but you are getting instructions to open Finder, which is a MacOS application. When you are reading a knowledge base article on this site, you can choose your operating system and Firefox version in the right column. Hopefully that will help.

If you are using Windows 7, you may need to make hidden files and folders visible in order to access the AppData folder. I'm on XP at the moment, so I can't describe exactly how to do that, but if you can call up the Tools menu, it's under Tools > Folder Options > View tab.

You are saying that I can call up Tools menu, it's under Tools > Folder Options > View tab. Where do I find the Tools menu? I cannot bring up Firefox to gain access to the Tools menu. All this is above my head, please can you explain it all in easy steps?

Two different Tools menus...

Firefox Tools menu
The Tools menu is on the classic menu bar. If you have the orange Firefox button at the top left, you can display the classic menu bar by tapping the Alt key or pressing the F10 button.

Windows 7 View Options
Open Windows Explorer (for example, hold the Windows button and press the e key). If the menu bar is not displayed, find the Organize button toward the upper left.

Organize > Layout > Menu bar

then

Tools > Folder Options > View

"Show hidden files, folders, and drives"

Hope that helps.