The scroll's thumb is fixed and cannot be moved when dragging it up and down

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox Beta 59.0b4 and up
• Snooze Tabs 1.0.21

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest Snooze Tabs add-on version (https://669-15314951-gh.circle-artifacts.com/0/tmp/circle-artifacts.kZdit8p/addon.xpi) loaded as a temporary add-on.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisite and navigate to a website.
2. Click the "Snooze Tabs" button from the toolbar.
3. Click the "Pick a Date/Time" option.
4. Click the "customizable time drop-down" field.
5. Click and drag up and down the scroll's thumb and observe the behavior.

[Expected result]:

• The scroll thumb moves up/down within the scrollbar.

[Actual result]:

• Nothing happens, the scroll's thumb is unmovable.

[Notes]:

• The issue is not reproducible if using the scroll buttons and mouse wheel.
• Attached a screencast of the issue:

[lmorchard]

I can reproduce this, but haven't figured out what could be causing it

[johngruen]

I'll poke at this for a minute and see if i can't figure something out.