Bugzilla

**Note**
* This issue not reproducible on Chrome.

**Affected versions**
* Fx 89.0b4
* Fx 89.0b4 DevEd
* Fx 90.0a1

**Affected platforms**
* macOS 10.15
* Windows 8.1 x64
* Ubuntu 20.04 x64

**Preconditions**
*Have a lots of folders created in Outlook, in order to expand the "All folders" dropdown and to have there a scroll bar.

**Steps to reproduce**
1. Launch Firefox.
2. Access the following website: https://outlook.live.com/  and login.
3. Click on search bar from outlook page in order to extend the left part of this bar, then click on a down arrow to expand the "All folders" list.
4. Observe the scroll bar on this dropdown list.

**Expected result**
*  The background of this scroll bar is not transparent. 

**Actual result**
* The background of this scroll bar is transparent. 

**Regression range**
* I will come back with regression range ASAP.

**Additional notes**
* See the attached screenshot.
* Reproducible with pref widget.non-native-theme.enabled = true and false too and pref browser.proton.enabled =true and false too.

**Note**
* This issue not reproducible on Chrome.

**Affected versions**
* Fx 89.0b4
* Fx 89.0b4 DevEd
* Fx 90.0a1

**Affected platforms**
* macOS 10.15
* Windows 8.1 x64
* Ubuntu 20.04 x64

**Preconditions**
*Have a lots of folders created in Outlook, in order to expand the "All folders" dropdown and to have there a scroll bar.

**Steps to reproduce**
1. Launch Firefox.
2. Access the following website: https://outlook.live.com/  and login.
3. Click on search bar from outlook page in order to extend the left part of this bar, then click on a down arrow to expand the "All folders" list.
4. Observe the scroll bar on this dropdown list.

**Expected result**
*  The background of this scroll bar is not transparent. 

**Actual result**
* The background of this scroll bar is transparent. 

**Regression range**
* I will come back with regression range ASAP.

**Additional notes**
* See the attached screenshot.
* Reproducible with pref widget.non-native-theme.enabled = true and false and pref browser.proton.enabled =true and false.