First of all, you're using Firefox 34.0.5 as your details say.

This addon called "British English Dictionary" is the most used addon currently.

Reply if this doesn't help.

Naeem said

First of all, you're using Firefox 34.0.5 as your details say. This addon called "British English Dictionary" is the most used addon currently. Reply if this doesn't help.

Naeem, Thank you, point taken about the version number - I simply saw the download page state that each of the dictionaries I tried said "Not available for 31.1.0". What actually happens is that any of the dictionaries I download are not recognised by Thunderbird. The spell-check pane appears, but does not highlight any misspelt word nor give any suggested replacement. I have found a dictionary downloaded to "My documents/ Downloads" but when I double click it the system cannot open an "xpi" file. Is this because I do not have Windows XP but Windows8. I also checked the download site for "Other versions" which all say not suitable for my system ? I fearI am confused! PS the spell checking has just worked on this message!

If opening an xpi file solves your problem, then on your browser, drag and drop the xpi file.. This will start an addon installation procedure...

Naeem said

If opening an xpi file solves your problem, then on your browser, drag and drop the xpi file.. This will start an addon installation procedure...

Thank you again, but, sorry, where do I drop the xpi file, please?

cjoldvet said

Thank you again, but, sorry, where do I drop the xpi file, please?

On any firefox tab. If for some reason that does not work, open up the addons tab (type about:addons in address bar and press enter) and drag & drop it on extensions panel...

Hope that helps..