Tested with:

Release 87
Beta 88.0b5
Nightly 89.0a1 (2021-03-31)

Tested on:

Windows 10
Windows 7 (Low Resolution)
MacOS 10.15

Preconditions:

In about:config, set browser.proton.enabled = true

Steps:

1. Launch firefox and open some tabs.
2. Close few tabs.
3. Open the Hamburger Menu and go to History -> Recently Closed Tabs
4. Close the hamburger Menu.
5. Open the Hamburger Menu and go to History -> Recently Closed Tabs again.

Actual result:

The toolbar separator between "Recently Closed Tabs" header and the recently closed tabs list is not displayed anymore after the user access the "Recently Closed Tabs" submenu for the second time and it appears again only after a browser restart.

Expected result:

The toolbar separator between "Recently Closed Tabs" header and the recently closed tabs list should not disappear when the user access the submenu for the second time.

Note 1: This is reproducible also for the Recently Closed Windows.
Note 2: This is not reproducible without Proton pref enabled.

Regression window:

https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ee0e3258730d5d39f41d66609961c4e59b12ec65&tochange=d12d902f1831560219306df6fc779c86d39ea363

The toolbarseparator is added the first time a subview is opened. However for subsequent times the header already exists and we bailed out early without adding the separator.

Backed out changeset 55776bb2f806 (Bug 1702200) for causing failures in nsSocketTransport.
Backout link: https://hg.mozilla.org/integration/autoland/rev/aa4f9e45c77b990601611f7252f5097fd810a672
Push with failures, failure log.

https://hg.mozilla.org/mozilla-central/rev/27f4ee87599b

Verified - Fixed in latest Beta 93.0b4 (build id: 20210912185727) and latest Nightly 94.0a1 (build id: 20210913213224).