I don't know what that script does or why it is loading in your Firefox. AVG seems to be detecting it, too: Threat:other:Malware-gen[Trj] | AVG.

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 those might be causing the problem.

If Firefox is not running: Hold down the option/alt 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)

When I switched back into firefox a couple of hours later the 'rain' of malware infection attempts had ceased.

So I imagine firefox had taken action themselves to deal with the problem.

Thanks for your help.