Bugzilla

**Note**
*note1

**Affected versions**
*version1

**Affected platforms**
*platform1

**Steps to reproduce**
1. Open firefox.
2. Open a few tabs until the tab navigation appear.
3. Focus on the last tab.
4. From the last tab open a new one with middle click.
5. Observe the size of the focused tab. 

**Expected result**
* Tab should keep it's size.

**Actual Result**
* Tab size is increased by a few pixels.

**Regression range**
* Will provide one ASAP.

**Additional notes**
* Please look at the added attachment for better understanding this issue

**Affected versions**
* 89.0a1(2021-03-29)

**Affected platforms**
* Ubuntu 
* MacOS
* Windows

**Steps to reproduce**
1. Open firefox.
2. Open a few tabs until the tab navigation appear.
3. Focus on the last tab.
4. From the last tab open a new one with middle click.
5. Observe the size of the focused tab. 

**Expected result**
* Tab should keep it's size.

**Actual Result**
* Tab size is increased by a few pixels.

**Regression range**
* Will provide one ASAP.

**Additional notes**
* Please look at the added attachment for better understanding this issue