What Firefox plugin or software can open a file from the clipboard?
I've asked this on Stack Overflow: How can I start a Windows GUI program using PHP? – but it seems hard to do it only using PHP code.
Not quite sure what you mean but there's "Launch Clipboard" add-on for FF.
You can use keyboard shortcut to launch FF with filename contained in clipboard. AutoHotkey http://www.autohotkey.com/. Create "*.ahk" file anywhere and place smth like this into it:
#H:: clipboard = %clipboard% EnvGet, programs, ProgramFiles(x86) StringReplace, programs, programs, \, \\, All Run, "%programs%\\Mozilla Firefox\\firefox.exe" file:///%clipboard% return
Run the script (H icon will appear in tray).
The script above will open things like "c:\docs\file.html" (contained in clipboard) in FF when you press Win+H (#H). Look through autohotkey documentation to define any other hotkey. You can also extend the script's functionality to store clipboard into a file and open it with FF.