Firefox doesn’t provide an easy way to browse, not to mention edit, its saved form data. However, they are simply stored in a plain SQLite database, so they are quite easy to access using an appropriate third party tool.
The form data are stored (on Windows) in %appdata%\Mozilla\Firefox\Profiles\[your FX profile]\formhistory.sqlite
On Linux and Mac OS, I’m not sure where this folder is located but the same principles apply: find that formhistory.sqlite file
Then you can open this file using any SQLite database browser, for instance DB Browser for SQLite, which is free and open source, and available on Windows, Linux and Mac OS. Using it should be quite straightforward if you have ever just touched some database software. The data of interest are in the table moz_formhistory, which is structured as follow:
– fieldname (= name of the form field, so if a form contains a field named “Email”, then Firefox will suggest all the values you already used for such a field)
– timesUsed (I believe this is used to prioritize the suggested values)
– firstUsed (timestamp of the first time the value was used)
– lastUsed (ditto for last time)
– quid (this one I have not idea)
Note that you can open the database for viewing while Firefox is running, but if you want to edit if you should first close Firefox, then open the database and do your stuff, then save, then re-open Firefox. Otherwise, you’ll probably lose your modifications.