AdGuard cannot be used on localhost

### see also

https://github.com/AdguardTeam/AdguardFilters/issues/51022

### Prerequisites

Please answer the following questions for yourself before submitting an issue. **YOU MAY DELETE THE PREREQUISITES SECTION.**

- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed

### Issue Details

- **AdGuard version:**
latest available
- **Browser and version:**
Firefox 74.0
- **Operating system and version:**
-

### Expected Behavior

One can enable AdGuard for localhost to try out whether a development page would work fine later.

### Actual Behavior

AdGuard refuses to do its job.