I am using a UK keyboard and can type French letters using the normal 3rd level key combo in the word processor programs.
Can anyone explain why I can not type the French letters using the same keys in internet forms using firefox?
Because the ability isn’t built into Firefox 
How about this add-on:
https://addons.mozilla.org/en-US/firefox/addon/easyaccent
?
Hi Thanks for this link and it works just fine…
No problem, glad it worked for you 