The Download Squad dug out this gem from the Mozilla Knowledgebase.
Firefox’s integrated spell checker is extremely handy for avoiding an embarrassing typo in an email, blog post, or Twitter update, but after a while, the light red underline is easy to ignore. Fix that with a quick about:config tweak as follows:
- Enter About:config in the Firefox address bar. If it pops up an alert, tell it that you know what you’re doing and proceed.
- Search for ui.SpellCheckerUnderlineStyle. If you find anything, skip to step 5.
- If your system is like mine, you found nothing. That means you have to add this configuration value. Right-click the main portion of the window and select New > Integer.
- Now, enter ui.SpellCheckerUnderlineStyle to create a setting by this name.
- Now you can set the value of this setting to one of several options: 0 for no highlighting, 1 for a dotted line, 2 for long dots, 3 for a single straight line, 4 for a double underline (like you see above – that’s my pick), and 5 for a squiggly line, which is the default