What you want is something like
<input type="submit" accesskey="F"/>
Now, when the users hits Alt + F (or Alt + Shift + F on Firefox) it will be as though he had clicked on the submit button.
EDIT: to use Alt + F on firefox too see this topic: http://forums.mozillazine.org/viewtopic.php?t=446830
You can change it back to how it was using about:config, change:
ui.key.chromeAccess to 5
ui.key.contentAccess to 4
In Opera, press Shift + Esc. That brings up the access key list for the current page. Then you can press any number from the access key list to follow the respective link. Users can change this keyboard shortcut under Tools > Preferences > Mouse and keyboard
.