If you start Firefox up in Safe Mode, does spell check work better there? It is probably an add-on of some sort interfering with normal operation.

That certainly seems to have cured it, now I will just try to track down the guilty party. Many thanks for going straight to the heart of the problem.

so what's the solution to this? run FF in safe mode always?

No, once you've run in safe mode, you can disable your extensions one-by-one until you find the culprit.

See also:

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I found that the culprit was an extension I really loved called Instaclick. It worked fine until there was an upgrade of firefox then this happened and it seemed to me it was the least suspicious extension but disabling it cured the problem. Hopefully I will be able to use it again sometime in the future.

The new instaclick update claims to fix this but I still have spelling suggestions accumulating in the context menu.