Could you try it this way, either:

• type or paste about:addons in the address bar and press Enter

• In the search box on the Windows 7 Start menu, type or paste
  
  firefox.exe "about:addons"
  
  and press Enter

If the page is still blocked, can you access it in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Can you get to the Add-ons page?

By the way, if you are investigating potential malware add-ons, you might need to start with the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help surface innocent sounding bundle items that snuck in with some software you agreed to install. Take out as much trash as possible here.

Do you see any error messages in the Browser Console (Firefox/Tools > Web Developer)?

• https://developer.mozilla.org/Tools/Browser_Console