"Yahoo" - what infected you? Email, some toolbar - what? Yahoo isn't malware. Do you have the home page set to about:blank?

No telling. Possibly in about:config - search for yahoo.

If that doesn't do it, uninstall Fx & clean install new copy.

Do you have some sort of yahoo toolbar or such? Did you use some sort of webmail "helper" or anything like that? Yahoo doesn't normally do "drive-by" infections, so it's a bit strange.

Aye,

It's likely that something 'innocent' had a small print 'by default we'll install this bit of software which you may not want, but we'll make some revenue from.

Fine - I was trying to get on with the job, and not paying attention.

Now - given that there is some unwanted behaviour in Firefox, I expect to have some configuration tools which provide a transparent view of the many options, potential extensions and so forth which are in effect. There are no extensions, add ons, toolbars, helpers, or anything related to yahoo. Yet it remains.

I'm using FF30, from June, and ths has persisted across previous upgrades.

As I said - I write software for a living, but on grown up computers, where our users are supposedly pretty dim, and need protecting from having to spend time in diagnosing faults, so we build adequate tools.

I'm a big fan of FF, and open source, which is why I'm miffed that this adware is so successfullly hidden.  

'Re install' - like the PC geeks answer 'Re boot the PC'

Looks like my only option then.

Thanks for the input.

Before you go and reinstall everything, try this:

You can delete a possible user.js file and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release. You can get to the profile folder below:

• Help > Troubleshooting Information > Profile Directory > Show Folder

or

• Go to about:support and click Show Folder next to Profile Directory.