'new tab' system icon is stretched when tab manager is displayed

[Phillip-Drake]

Describe the bug

'new tab' system icon is stretched when tab manager appears, likely due to a contorted aspect ratio.

To Reproduce

1. Go to 'about:config'
2. Create variable 'gnomeTheme.systemIcons'
3. Set new variable to 'true'
4. Create tabs until tab manager appears

Expected behavior

'new tab' system icon displayed as expected :

Screenshots

As can be seen, the 'new tab' system icon is now stretched.

Setup information

• OS: Fedora 39
• Installation method: Curl script
• Firefox version: 122
• Theme version: master
• Optional features enabled: gnomeTheme.systemIcons, gnomeTheme.tabsAsHeaderBar

Additional context

Issue is agnostic of gnomeTheme.tabsAsHeaderBar being enabled, but is not reproducible when gnomeTheme.systemIcons is disabled. I am using Adwaita icon pack

[Phillip-Drake]

Created pull request for master branch including a fix for this bug.