To make sure spell checking is enabled with the built-in dictionary, see: How do I use the Firefox spell checker?

If you were used to a different interface (not just the squiggly lines and right-click context menu), it's possible you were using an add-on that enhanced the built-in features. Do you have any memory of that?

Wasn't using anything other than the standard FF spell checker. Followed the link you sent and confirmed the correct box IS checked on the menue (spell checker didn't pick up "menu" spelt wrong !!) so am at a bit of a loss now.

Says "Automatic spell-checking is only turned on for text boxes containing more than one line, where you can enter as much text as you want."

Is that new or has it always been like that ?

"Always" is a long time... It has been that way for years, and you can change it here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste SPEL and pause while the list is filtered

(3) Double-click the layout.spellcheckDefault preference and change it from 1 to 2 to spell check single-line controls as well as multi-line controls.

Note: this should turn spell checking on by default, but sites can still turn it off.