Since version 3 of Firefox it has had a built in spell checker. This is handy if say you use your browser to write blogs or posts online for example. The problem is when you download the browser it doesn’t come with any dictionary installed so it has the ability to spell check things but it doesn’t have anything to check it against.
Mozilla Thunderbird also has the same problem as Firefox.
The spell checker uses the same technique used in Microsoft Word. It highlights an incorrect word by underlining it with a red dotted line.
To get spell checker head over to the dictionary page from Mozilla. Download your language(s). You only need to download the dictionary once if you want to use spell check in Firefox and Thunderbird.
Tools > Add-ons menu and a window should appear. Drag the dictionary files into the window and it should ask for confirmation to install. Wait the timeout delay (usually 3 seconds) and press “Install”. It will then ask you to restart. You can continue using your application but the spell check feature won’t be enabled till you restart or close and re-open Firefox/Thunderbird.
This process works for both products.