When you have entered a value into a form, Firefox will remember it and suggest it the next time you use the form. When such a value is incorrect or for some other reason not relevant anymore you probably want to delete it. Very simple but not very well documented. Here is how it is done:
When I installed Open SuSE 11.4 yesterday I thought that the fonts used in GTK applications like Firefox and Thunderbird were really ugly. In my KDE settings I have specified that I want it to use my KDE fonts in GTK applications but this setting seems to have no effect. To fix this I installed gnome-control-center from the repository and changed the fonts to 8 point Liberation Sans and Liberation Mono. Now the GTK applications look just as good as the KDE ones.
How do I get Firefox to open mailto-links with KMail and use KMail from Tools/New Message?
For some reason this IMO important configuration setting hasn't made it into the Preferences dialog but it is available: