The add-on does not import the old configuration after migrating from MV2 to MV3

**System**

* OS name: Windows
* OS version: 7 Pro
* Browser name: Firefox
* Browser version: 148 (fork restoring support for legacy systems)
* Extension version: 5.1.0

**Bug description**

I’ve noticed that version 5.1.0 can not process the settings from version 5.0.0, which means that someone acting on muscle memory might accidentally cause themselves harm – for example, by deleting all cookies whilst clearing everything else instead.

At least in Firefox 147 and 148 in the Windows 7 fork (testing on Chromium is more difficult with a real ID rather than one from an unpacked extension or archived signed ‘crx’ files).

I’m not sure whether you intended to start from scratch, or whether this is a bug specific to the Firefox fork that doesn’t affect anyone using standard versions or those not built for Windows 7.

**Logs**

```
// REPLACE WITH LOGS
```