The Logitech MX3100 (LX 700/MX 3000 keyboard) has special functions for the F keys. Strangely, I never had a problem on my old installation. But now that I’m using the latest version of their SetPoint software, it seems to override Firefox’s built-in keyboard shortcuts; namely, F5 for refresh. Instead, it was showing “Undo” (in ugly green text at the bottom) and doing apparently nothing. I always use CTRL+Z for undo, so that’s no use to me. So I went into the SetPoint software and tried to find a way to leave things as they would be without SetPoint. Unfortunately, I couldn’t find a way. But I searched the web a bit and realized that CTRL+R does the same thing as F5 (refresh the current page in Firefox). So I set F5 in SetPoint to do a keyboard combo (macro): CTRL+R. Works like a charm.