From MozillaZine Knowledge Base

Introduction

This article may be also be applied to users that use Firefox installers; it is unsure if the Mozilla Firefox uninstaller that is provided with the installer preserves your plugins directory.

When not installing Firefox from an installer, removing the Firefox installation directory and then extracting, instead of overwriting the Firefox folder, is a good idea. However, it removes your plugins in your "plugins" folder each time, unless you're careful. Fortunately, the plugins in your "plugins" folder may be moved elsewere so that they don't reside under your installation directory and so to make your manual uninstalling of Firefox easier.

Firefox searches a few locations for plugins such as Macromedia Flash and Java. One of these locations is in a "Plugins" folder a parent folder of your Mozilla profile, if one exists. This search is done even if you have no legitimate Mozilla profile. ç

The Workaround

To trick Firefox into searching your folder, locate where the Mozilla profile container folder should be (for example, "C:\Documents and Settings\<Windows login/user name>\Application Data\Mozilla\Profiles\"). If this folder does not already exist, create it. Now there should be a path that resembles ".../Mozilla/Profiles" (where "..." is the actual path where "/Mozilla/Profiles" exists). Create a folder in ".../Mozilla" named "Plugins" so that now there is a "C:\Documents and Settings\<Windows login/user name>\Application Data\Mozilla\Plugins\" folder.

If Mozilla is installed, make sure that this is not the actual Mozilla installation directory. It should have "Profiles" folder directly underneath it; if you do find a "profiles" folder, but it is a ".../defaults/profiles" folder, then that is your installation directory and you will have to search further.

Now locate where Firefox is currently storing your plugins. It will be under a "plugins" folder in your Firefox installation directory (for reference, the default directory for a Windows Firefox installer is "C:\Program Files\Mozilla Firefox\plugins").

On Mac OS X, your Firefox folder is usually in your Applications directory. However, to access it, you must Ctrl-Click the Firefox folder and select "Show Package Contents". Afterwards, you should be able to browser to your plugins folder, which should be located in "/Applications/Firefox.app/Contents/MacOS/firefox/plugins".

If all else fails, search for a "firefox" directory with a "plugins" folder on your computer using a file find utility. Make sure there is a "plugins" folder underneath the "firefox" folder, as Firefox also stores its profile in a directory named "Firefox". If you have plugins, this "plugins" folder should have more than just a "npnul32.dll" file or the likes.

Now cut all the plugins in the real Firefox plugins directory (without "npnul32.dll" file (file name may differ on Linux) -- this file might be updated between Firefox builds, so it should be removed when manually uninstalling Firefox) and paste them into the fake plugins directory.