Can I keep stored data for a Greasemonkey script even when it's uninstalled?

Question

I've written a Greasemonkey script that saves preferences using GM_SetValue. Sometimes, if there's an issue with the script, I advise users to reload the script by removing it and re-installing. This blows away the stored values for the script, which is very inconvenient for the user.

Is there a better way to store this information? The state I need to store is per-page, using the URL as a key for GM_SetValue, if that makes any difference.

Answer 1

Yes, you can copy the SQLite file that stores GM_setValue data and preserve that information.

To find the data:

1. Go to your Firefox profile folder.
2. Enter the gm_scripts folder therein.
3. The appropriate file will be named based on the script name and will have the extension .db.
   For example if the script is named:
   // @name _Zombie GM_setValue fun
   Then the SQLite file will be named:
   _Zombie_GM_setValue_fun.db

If you move or copy this file to a safe place, then do whatever to the script, then copy the file back; your data will be preserved. (As long as you don't change the script's @name or @namespace.)
You don't have to close Firefox while you do this, but I would -- to guard against edge-case mishaps.

Example script:

// ==UserScript==
// @name     _Zombie GM_setValue fun
// @include  https://stackoverflow.com/questions/28498610/*
// @grant    GM_getValue
// @grant    GM_setValue
// ==/UserScript==

var lastVal = GM_getValue ("LastValue");
var newVal  = prompt (
    'The last value was "' + (lastVal || "{not set}") + '". Enter a new value:',
    ''
);
if (newVal)
    GM_setValue ("LastValue", newVal);

Test sequence:

1. Install the script.
2. Reload this very page (stackoverflow.com/questions/28498610/).
3. You'll see The last value was "{not set}". Enter a new value:.
4. Enter Save me.
5. Reload the page.
6. You'll see The last value was "Save me". Enter a new value:
7. Copy _Zombie_GM_setValue_fun.db to a safe place.
8. Uninstall the script and (optionally) restart Firefox.
9. Reinstall the script.
10. Load the page and you'll see "{not set}" as in step 3.
11. Copy the saved _Zombie_GM_setValue_fun.db back to the gm_scripts folder, overwriting the newer _Zombie_GM_setValue_fun.db if it is present.
12. Reload the page you'll see Save me as in step 6.