Tested with:

Nightly 88.0a1 (2021-02-24)

Tested on:

Windows 10
Ubuntu 20.04

Preconditions:

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

Steps:

1. Launch firefox and sign in into FxA.
2. Click on the sync now option and wait few seconds.
3. Observe the Sync Now option.

Actual result:

The "Last synced ..." time from the FxA submenu always shows 1 second ago no matter how much time has passed from the last sync. When clicking on sync now, for a second, it shows the correct time.

Expected result:

The "Last synced ..." time should be updated accordingly.

Note: The issue is reproducible with and without Proton enabled.

I spoke with UX, and since the label does no auto-update, we're going to remove it.

The button should say "Sync now" without an icon, and when it is currently syncing, we update that text to "Syncing..." with the blue sync icon.

This patch:
-Maintains consistency between the fxa and remote tabs' panels "sync now" buttons
-Removes a fluent string no longer in use
-Restores tooltip functionality

Backed out for causing failure at browser_sync.js.

Backout link: https://hg.mozilla.org/integration/autoland/rev/cd2a2a5321a7818e764a9f674e7427ec0c91bcbd

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=aPw7w2JaQsuA_y03EjUf5g.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel%2Crunnable&searchStr=linux%2C18.04%2Cx64%2Ctsan%2Copt%2Cmochitests%2Ctest-linux1804-64-tsan%2Fopt-mochitest-browser-chrome-e10s%2Cbc2&revision=962540b712f9aa12ec353838105cb19a3d3bd93c

Failure log: https://treeherder.mozilla.org/logviewer?job_id=331437389&repo=autoland&lineNumber=2781

Backed out changeset 7692908f97e7 (Bug 1694678) for causing bc failures in browser_sync.js

Backout link: https://hg.mozilla.org/integration/autoland/rev/fc9d862fb7bb7aed5d002d4f6888f43d82e34cde

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&resultStatus=testfailed%2Cbusted%2Cexception&revision=7692908f97e748bddda32ccbb23b1de15283d27b&selectedTaskRun=cAtt6XmtTUGeMg8cTFPf4w.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=331781222&repo=autoland&lineNumber=2300

There are also the following failures from this patch:
https://treeherder.mozilla.org/logviewer?job_id=331783115&repo=autoland
https://treeherder.mozilla.org/logviewer?job_id=331787769&repo=autoland

Backed out for failures on browser_sync.js

backout: https://hg.mozilla.org/integration/autoland/rev/3d514b514408ebeec5a009b400c0c0a02742e9a6

push: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=I7pJsz82Q8KWpspGg0MMxA.0&revision=44ff4cb38ce187b7f0f5950b32f2b72c8d200c24&group_state=expanded

failure log: https://treeherder.mozilla.org/logviewer?job_id=331856071&repo=autoland&lineNumber=3325

[task 2021-03-03T07:42:53.451Z] 07:42:53 INFO - TEST-PASS | browser/base/content/test/sync/browser_sync.js | PanelUI-fxa-menu-setup-sync-button is hidden -
[task 2021-03-03T07:42:53.453Z] 07:42:53 INFO - Buffered messages finished
[task 2021-03-03T07:42:53.453Z] 07:42:53 INFO - TEST-UNEXPECTED-FAIL | browser/base/content/test/sync/browser_sync.js | expected avatar URL to be url("chrome://browser/skin/fxa/avatar.svg"), got none -
[task 2021-03-03T07:42:53.453Z] 07:42:53 INFO - Stack trace:
[task 2021-03-03T07:42:53.453Z] 07:42:53 INFO - chrome://mochikit/content/browser-test.js:test_ok:1331
[task 2021-03-03T07:42:53.453Z] 07:42:53 INFO - chrome://mochitests/content/browser/browser/base/content/test/sync/browser_sync.js:checkFxAAvatar:584
[task 2021-03-03T07:42:53.453Z] 07:42:53 INFO - chrome://mochitests/content/browser/browser/base/content/test/sync/browser_sync.js:test_ui_state_signedin:161
[task 2021-03-03T07:42:53.454Z] 07:42:53 INFO - Not taking screenshot here: see the one that was previously logged
[task 2021-03-03T07:42:53.455Z] 07:42:53 INFO - TEST-UNEXPECTED-FAIL | browser/base/content/test/sync/browser_sync.js | expected avatar URL to be url("chrome://browser/skin/fxa/avatar.svg"), got none -
[task 2021-03-03T07:42:53.455Z] 07:42:53 INFO - Stack trace:
[task 2021-03-03T07:42:53.456Z] 07:42:53 INFO - chrome://mochikit/content/browser-test.js:test_ok:1331
[task 2021-03-03T07:42:53.456Z] 07:42:53 INFO - chrome://mochitests/content/browser/browser/base/content/test/sync/browser_sync.js:checkFxAAvatar:584
[task 2021-03-03T07:42:53.456Z] 07:42:53 INFO - chrome://mochitests/content/browser/browser/base/content/test/sync/browser_sync.js:test_ui_state_signedin:161
[task 2021-03-03T07:42:53.457Z] 07:42:53 INFO - GECKO(1855) | [Parent 1855, Main Thread] WARNING: NS_ENSURE_TRUE(weakFrame.IsAlive()) failed: file /builds/worker/checkouts/gecko/layout/xul/nsXULPopupManager.cpp:1068
[task 2021-03-03T07:42:53.458Z] 07:42:53 INFO - GECKO(1855) | [Parent 1855, Main Thread] WARNING: '!mPresContext', file /builds/worker/checkouts/gecko/dom/events/UIEvent.cpp:138
[task 2021-03-03T07:42:53.458Z] 07:42:53 INFO - TEST-PASS | browser/base/content/test/sync/browser_sync.js | fxa label has the right value -

https://hg.mozilla.org/mozilla-central/rev/3e2fdbe6d3f2

Verified - Fixed in latest Nightly 88.0a1 (build id: 20210315214853) both with proton pref on and off, using Windows 10, Ubuntu 18.04 and MacOS 10.15.