Old confirmation bar remains displayed when installing the latest Snooze Tabs version

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox 58.0.2 and above
• Snooze Tabs 1.1.0

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Snooze Tabs version from Test Pilot is installed.
• "Ask for confirmation when snoozing tabs" checkbox is checked.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to a website (e.g. "www.wikipedia.org").
2. Click on the "Snooze tabs" toolbar button.
3. Click on one of the snooze options (except "Pick a Date/Time").
4. Go to "about:addons" and drop the latest Snooze Tabs .xpi (https://github.com/bwinton/SnoozeTabs/releases/tag/1.1.0) in the content area.
5. Go to the website from step 1 and repeat steps 2&3.
6. Observe the confirmation bar.

[Expected result]:

• Only the confirmation bar triggered at step 5 is displayed.

[Actual results]:

• The confirmation bar from step 5 overlaps the old one, from step 3.

[Notes]:

• Attached a screencast of the issue: