Note that if you restore your old add-ons, you will very likely run into the same problem that made you reset Firefox in the first place. The following directions assume you haven't installed any new add-ons in the current profile. If that's not the case, don't follow them; manually re-install your old add-ons from the Firefox Add-ons site instead.

1. Type about:support into the address bar and press Enter.
2. Click the Show Folder button. A Windows Explorer window with your current profile opens.
3. Right-click the Firefox taskbar icon and choose either Close Window or Close All Windows.
4. Open the Old Firefox Data folder on your desktop.
5. Right-click the extensions folder and choose Copy.
6. In the window with your current profile, click the Organize button and choose Paste. Windows Explorer will ask if you want to merge the folders: click Yes.
7. Repeat the above two steps with the following files: extensions.sqlite, extensions.json and extensions.ini. If you're prompted whether to replace any existing files, answer Yes.

The extensions.sqlite file is no longer used in the current Firefox release, so is likely not present in the Firefox profile folder.

To avoid confusion: there is a difference between plugins that extends support for file types that Firefox can't handle internally and extensions that add extra features to Firefox.

• https://support.mozilla.org/kb/Customizing+Firefox+with+add-ons