The "Select email provider" button doesn't have any tooltip

[Softvision-RemusDranca]

[Affected versions]:

• Firefox 63.0.1 and above

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a new Firefox profile with the latest version of the "Email Tabs" add-on (0.1.7616) custom build.
• You are logged in your selected email provider.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Click the "Email-Tabs" toolbar button.
3. Hover over the gearwheel "Select email provider" button from the top-right side of the doorhanger.
4. Observe the behavior.

[Expected result]:

• A specific tooltip is displayed.

[Actual result]:

• No tooltip is displayed.

[Notes]:

• Attached is a screen recording of the issue:

[Softvision-RemusDranca]

I have verified this issue using the latest Email Tabs version (0.1.764) custom build, and the issue is no longer reproducible. Tested on Windows 10, Mac 10.13 and Arch Linux 4.16.6 x64.