It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to check if there is a compatibility update available.

If this hasn't helped then also try to delete the addons.sqlite file.

More recent threads about Selenium:

• https://support.mozilla.org/search?q=Selenium&w=2&a=1&updated=2&updated_date=06/01/2013&sortby=1

1) Delete all the files stored in your profile

    Help -> Troubleshooting Information -> Click on "Open Directory" under 
   Application Basics 

 This will open your profile directory
 Select all the files and delete

2) Download Selenium again from docs.seleniumhq.org/download/

With this procedure I could view the IDE in tools.