Does the adult site load as soon as you open Firefox? Or does it happen randomly after Firefox has been open for a while?

Also, try to Restore to the Default home page and see if that helps.

1. It happens randomly after opening Firefox.

2. Have restored the home page.

3. Anything else I can do - I really don't want this irritation.....

Do a malware check with a few malware scan programs. You need to use all programs because each detects different malware. Make sure that you update each program to get the latest version of the database.

http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware http://www.superantispyware.com/ - SuperAntispyware http://www.safer-networking.org/en/index.html - Spybot Search & Destroy http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page .......... See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and What to do when searches take you to the wrong search website

Many thanks!! I hope it helps