Tested with:

Release 88
Beta 89.0b8
Nightly 90.0a1 (2021-05-04)

Tested on:

Windows 10
Windows 7

Preconditions:

RTL build installed.

Steps:

1. Launch firefox (RTL build) and sign in into FxA using 2 devices.
2. Open more than 25 tabs on one device and sync tabs with another devices.
3. Open the hamburger menu, go to Firefox Account submenu and scroll down to the "Show More Tabs" option .

Actual result:

The "Show More Tabs" arrow points upwards instead of downwards.

Expected result:

The "Show More Tabs" arrow should points downwards in order to expand the list of synced devices.

Ni? @Tyler for prioritize this.

Before this patch, we were showing a downward-pointing expander icon by taking a
rightward-pointing one and rotating it 90deg. That's fine, except for RTL, where we
tend to mirror rightward-pointing arrows with a scale: -1 1; rule. Because of
order of operations, the scale rule is applied first, and the 90deg rotation
that occurs afterwards ends up pointing the scaled arrow upwards.

Instead of special-casing the scale rule for this expander icon, I've gone ahead
and switched it to use arrow-dropdown-16.svg and gotten rid of the rotation. This
way, even if we mirror across the y-axis with scale: -1 1, the appearance will
not change.

https://hg.mozilla.org/mozilla-central/rev/e69e2aecb3dc

Verified - Fixed in latest Nightly 90.0a1 (build id: 20210519214756) using Windows 10 and Ubuntu 20. The "Show More Tabs" arrow points accordingly, downwards, in RTL builds.