I am not able to get Firefox 17 to retain the option to check my spelling as I type in the context menu but the option is retained under options advanced browsing. If I want to verify the spelling of a word and use the spell checker I have to right click on the word in question then select check my spelling as I type in the context menu by placing a check mark by the option.
The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.
To Reset Firefox do the following:
Go to Firefox > Help > Troubleshooting Information.
Click the "Reset Firefox" button.
Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
Firefox will open with all factory defaults applied.
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"