GFork

This script simply removes spellcheck="false" wherever it is found in a document. It also adds an item to the monkey menu to remove those again if they are introduced in the process of using the page.

Test document: http://www.jeffersonscher.com/gm/nospell.html

Note: This script simply restore default Firefox behavior, so text inputs are not marked for spell checking but you can turn on spell checking in such inputs using the right-click context menu.