Do you have a dictionary installed and selected?

You can see which dictionary is selected if you right-click in a text area and open the Languages submenu. At least one dictionary should be installed and have a check mark to indicate that it is selected.

• Open the "Add Dictionaries" link to install a dictionary if you do not have one.
• Make sure that [X] "Check Spelling" in the right-click context menu is check-marked.

You can enable or disable spell checking globally:

• Tools > Options > Advanced : General: Browsing: "Check my spelling as I type"

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

looks like installing dictionary has fixed it, why would it come preinstalled without one and have spellchecking on as default?

What Firefox locale do you use?

Only a few locales come with a dictionary because of license reasons. With locales that do not come with a dictionary you need to install the dictionary yourself.