I'm using Ubuntu 14.04 and its built-in version of Firefox, which is currently version 30.0.
On some web pages spellchecker is working properly, but on many it does not. If I right-click in a text box the "Check Spelling" option is checked, but then if I go to the Languages entry that appears right below that, "English (United States)" doesn't have a dot next to it, meaning it's not selected, and I'm not getting any spellchecking functionality. It's the only language available in the list. If I then click on "English" the spell checker starts working.
So, when using text boxes on some websites "English" is selected and the spellchecker works, but on many many others, including here, "English" isn't selected and spellchecker doesn't work until I select "English" manually.
Does anyone know why this is? Doing a Google search for the issue it seems like Firefox has a history of spellchecking issues.