now I have to remember everything I did
If by plug-in you mean extension (I refer to plug-ins such as things like Flash, Shockwave, WMP embeds, etc)....the settings for them should be stored in your profile folder, which for WinXP is located around C:\Documents and Settings\<User Name>\Application Data\Mozilla .
For my thumb-drive, I usually just copy & paste that folder so I can use Firefox on any PC, with a little batch file telling the browser to load profile from a defined directory, rather then the default location.