It appears I have found the culprit . . . .
It seems to be a firefox add-on called adblock plus popup addon 0.9 - bizarre as this has been installed a while so it must be an update that's affected it. I disabled all add-ons in FF the re-enabled them one by one checking for that behaviour as I went along and this one's 'it' - consider me 'solved' for now and if anything else develops I'll post back . . . .