The `extensions.fxmonitor.warnedHosts` pref is not working properly

[GeorgiuCiprian]

[Affected versions]:

• Release 63.0.1
• Beta 64.0b7
• Nightly 65.0a1

[Affected Platforms]:

• Windows 10 x64
• macOS 10.13
• Ubuntu 16.04 x64

[Prerequisites]: Create the following prefs in about:config:

• extensions.fxmonitor.enabled (bool) = true
• extensions.fxmonitor.debug (bool) = true (restart the browser in order to apply the prefs)

[Steps to reproduce]:

1. Install the signed FM add-on v2.5 from here: https://drive.google.com/file/d/1P3STM5zoUstEGp1tPUhyBiCrhNb-C4ng/view?usp=sharing
2. Navigate to any websites that has breaches within 2 months.
3. Go to about:config and look for the extensions.fxmonitor.warnedHosts
4. Delete from that pref the sites visited on step 2.
5. Restart the browser.
6. Visit the same websites deleted at step 4.

[Expected result]:

• The doorhnger is displayed.

[Actual result]:

• The doorhanger is not displayed.

[Regression]:

• Not a regression.

[GeorgiuCiprian]

We have verified this bug on 63.0.1 and 64.0b7 and is not reproducible anymore on the 2.7 version off the add-on. Tested across platforms: Windows 10 x64, Windows 7 x86, macOS 10.13 and Ubuntu 18.04 x64.

During this verification we have uncovered another issue which is filed in #134. Closing this as verified fixed since it will be tracked separately.