NoSpellCheck

Chrome allows you to disable spellchecking on a per-field basis (right click -> Spell-checker options), however it defaults by spellchecking all single-line text input fields. Usually these fields are meant for email addresses or usernames, so you end up with a lot of red lines unless you manually whitelist all these fields. Firefox defaults to ignore these single-line input fields, allowing you to manually enable checking if you want.

This extension enables the Firefox spellchecking behaviour, only checking multiline textarea fields by default.

Known Issues:

If this extension is enabled, you cannot use the "Spell-checking options" to manually enable an input field. There is no way for the extension to know about this setting, currently, and there is no way to disable spellchecking while respecting Chrome's own settings. So if you rely on spellchecking for manually whitelisted text fields this extension will not work so great for you.

Source is available on Github: http://github.com/lsegal/NoSpellCheck