I guess the spellchecker went crazy because the wrong dictionary was selected. To select the right dictionary:

1. Right click on a text area and open the Languages sub-menu.
2. Make sure the required dictionary (English) has a tick mark against it.

If this does not help and you really wish to disable spell checking completely, please see these instructions: How do I use the Firefox spell checker?.

Thanks, yes that was the problem. For some weird reason English had disappeared, only French remained. Now fine.

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