I wrote my own Firefox extension:

None of the existing ones did quite what I wanted or had a single right-click menu entry. My little extension does what it says on the tin: It converts text to title case, ignoring some words that aren’t typically capitalized. It’s signed and everything (though that is an evil scheme).

The extension is not available in the Mozilla Add-Ons site as it’s about as tested as a wombat in space, but it works for me.

NO I AM NOT A DEVELOPER LEAVE ME ALONE