Sometimes it becomes hard to find things that otherwise seem blatantly obvious. I was searching on Google to backup the search engines my Firefox and realized that this information isn't easily reachable. Of course hardcore Firefox users would tell you this even when they're half asleep, but me; I don't fall in that category.
So here is what needs to be done. Get to the profile folder of Firefox, for typical installations it would be following on xp & Vista.
xp » C:\Documents and Settings\[UserName]\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default
Vista » C:\Users\[UserName]\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxxx.default
Or if you have setup your profile in another location using the ProfileManager command line option, you know where your Firefox profile is located. There is a folder called searchplugins which contains all the engines, one XML file per engine. Use your discretion to backup the engines, you may want to backup the entire folder or you may want to export/backup engines selectively.