Are you using the built-in Google search plugin or a custom Google search plugin? Normally, searches done with the built-in search plugin would not have a coded query on top of a regular query, but would look much more like this:

https://www.google.com/search?q=firefox+download&ie=utf-8&oe=utf-8

In case this is caused by an add-on, could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

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).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)

Dear jscher2000,

I tried your suggestion and the sma error appeared. I started in safe mode and then in teh serach bar typed in a search paramater which is "sample letter" and that same Error 403 came up.

btw. Thank youfor your speedy response. I hope we can work this out.

Is there any problem if you search directly on https://www.google.com/ ?

To investigate which search plugin you are using, you can open the Options page and check your default search engine setting. See: Change your default search settings in Firefox

If the "Restore Default Search Engines" button is enabled (text is black instead of grayed), try clicking that to see whether that changes how Google works.