When trying to activate Webgl on Firefox, I misspelled "Webgl" and wrote "Weblg".
I tried to remove it from about:config with delete key or seeing right mouse click options on it, but without success...
Googling about, I found that solution:
Close Firefox. Backup the prefs.js file located in your profile folder. Open the original prefs.js in a text editor. Remove erroneous key, and save.
Simply reset the entry that you want to delete in about:config and then restart firefox et Voilà !
I don't know how long this solution will remain valid but it works fine at least for now...