Bugzilla

**Affected versions**
* 88.0a1 (2021-03-14) 

**Affected platforms**
* Windows 10;
* with macOS 11.3, there is a shadow to the tooltip that helps out a bit;

**Steps to reproduce**
* enable proton prefs;
* OS theme set to dark;
1. Launch Firefox, set it on dark theme / OS theme set to dark;
2. Open at least 1-2 tabs;
3. Hover over tabs;

**Expected result**
* tooltip visible and differentiated with other elements;

**Actual result**
* lack of item shadow makes tooltip blend in with address-bar background;
* used colors are to close one to another(on the color spectrum) and appear as blended;

**Regression range**
* not a real regression, visible after 2021-03-04 / tabstrip-tooltip work / bug ;
* pushlog [URL](https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=5199ec2d73fa7daf423b72416be41383ac1042dc&tochange=95b6c826ad974a967364cb1f137bd8a766051d0a)

**Additional notes**
* :rtestard indicated to set as P3;
* attached screenshot to illustrate the issue.

**Affected versions**
* 88.0a1 (2021-03-14) 

**Affected platforms**
* Windows 10;
* with macOS 11.3, there is a shadow to the tooltip that helps out a bit;

**Steps to reproduce**
* enable proton prefs;
* OS theme set to dark;
1. Launch Firefox, set it on dark theme / OS theme set to dark;
2. Open at least 1-2 tabs;
3. Hover over tabs;

**Expected result**
* tooltip visible and differentiated with other elements;

**Actual result**
* lack of item shadow makes tooltip blend in with address-bar background;
* used colors are to close one to another(on the color spectrum) and appear as blended;

**Regression range**
* visible after 2021-03-04 / tabstrip-tooltip work / bug ;
* pushlog [URL](https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=5199ec2d73fa7daf423b72416be41383ac1042dc&tochange=95b6c826ad974a967364cb1f137bd8a766051d0a)

**Additional notes**
* :rtestard indicated to set as P3;
* attached screenshot to illustrate the issue.