Updated horizontal tabs fixes (uplift to 1.62.x)

[zenparsing]

Uplift of:

• Reduce the space above the tabstrip in Windows #22000
• Allow first tab to be selected at edge of screen in fullscreen #21989
• Fix tabstrip background color when themes are used #21969
• Remove corner rounding for tab hit test paths #21967

Pre-approval checklist:

• You have tested your change on Nightly.
• This contains text which needs to be translated.
  • There are more than 7 days before the release.
  • I've notified folks in #l10n on Slack that translations are needed.
• The PR milestones match the branch they are landing to.

Pre-merge checklist:

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.

Post-merge checklist:

• The associated issue milestone is set to the smallest version that the changes is landed on.