That's a massive number of add-ons you've got there, based on their names some may even be overlapping in what they do (and hence cause conflicts; double Flash is never a good thing for a start). Does the problem persist if you run just the bare browser with no add-ons? A reinstall might be necessary to make sure no extra load gets left behind.

Thanks. All of those add ons might not be enabled. Can't see where add ons are shown here. Will get to work on your suggestions shortly and see how it goes.

The Question Details area has the More System Details link that shows the add-ons you had at the time of reporting. It's to the right of the opening post and does discern between active and inactive add-ons, but that applies only to add-ons that Firefox has detected.

I've come to learn that they don't always uninstall cleanly, i.e. they may leave changes behind even if disabled or even uninstalled - that's why a fresh reinstall may be needed after a while. Although Refresh may accomplish much the same effect, it may be spoiled when the old profile data is reintroduced.

Appreciate that. Thanks.

Didn't manage to find the double flash items you referred to. Would appreciate your help in locating them.

No problem; I attached an image and marked the navigation and double Flash plugins in pink.

I also marked some add-ons I view as problematic in red: antivirus add-ons are usually more trouble than they are worth, interfering with standard behaviour while providing limited security at best. NoScript and other content blockers are more useful, as they will simply block unwanted content while leaving the browser to do its job (and external scanning will still take place).

Hola is a suspect service provider due to their disregard for their users: https://torrentfreak.com/hola-vpn-sells-users-bandwidth-150528/.

The Gmail Notifier seems to have some corruption in its name - either that, or there's just a symbol issue.

The more moving parts there are, the bigger the risk something goes wrong, so the great number of add-ons has risk potential.

Very many thanks. I disabled a quite a number of addons including duplicates and the problem is now solved. Number of addons is now much reduced. No idea which one, or ones, contributed to the solution just delighted to have it sorted. Thanks again.

Good to hear, you're welcome! Please choose one of the posts as the Solution so that others can find it more easily (it will show up under your starting post) and the thread can be marked as solved.

Sorry for delay in getting back to mark as solved. Was away. Disabling a large number of addons, and even deleting a good number, definitely solved the problem.