Let's start with: If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

When done, copy and post the results here.

https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

What code do you have in userChrome.css as that is still the most likely cause ?

You can try Restore Defaults on the customize page. You can try to open a new window.

You can clear the startup caches via the "Help -> Troubleshooting Information" (about:support) page to see if that has effect.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Ok I found a quick fix: changing Density to Compact in Customize. Changing it back to Normal causes the arrow to be visible for current session, but when I relaunch the browser it's gone again. Not sure if it should happen, but it works for me. Anyway here's the info you wanted because maybe there's another solution.

Let's start with: If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again. [v57+] Places Maintenance is built into Firefox. Type about:support<enter> in the address bar. You will find Places Database near the bottom. Press the Verify Integrity button. When done, copy and post the results here. https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

Thanks for answer. Here's the result:

> Task: checkIntegrity + The places.sqlite database is sane + The favicons.sqlite database is sane > Task: invalidateCaches + The caches have been invalidated > Task: checkCoherence + The database is coherent > Task: expire + Database cleaned up > Task: originFrecencyStats + Recalculated origin frecency stats > Task: vacuum + Initial database size is 20480KiB + The database has been vacuumed + Final database size is 20480KiB > Task: stats + Places.sqlite size is 20480KiB + Favicons.sqlite size is 15488KiB + pragma_user_version is 54 + pragma_page_size is 32768 + pragma_cache_size is -2048 + pragma_journal_mode is wal + pragma_synchronous is 1 + History can store a maximum of 139935 unique pages + Table moz_origins has 1654 records + Table moz_places has 31582 records + Table moz_historyvisits has 61011 records + Table moz_inputhistory has 129 records + Table moz_bookmarks has 104 records + Table moz_bookmarks_deleted has 0 records + Table moz_keywords has 0 records + Table sqlite_sequence has 1 records + Table moz_anno_attributes has 2 records + Table moz_annos has 184 records + Table moz_items_annos has 0 records + Table moz_meta has 7 records + Table sqlite_stat1 has 17 records + Index sqlite_autoindex_moz_origins_1 + Index sqlite_autoindex_moz_inputhistory_1 + Index sqlite_autoindex_moz_bookmarks_deleted_1 + Index sqlite_autoindex_moz_keywords_1 + Index sqlite_autoindex_moz_anno_attributes_1 + Index moz_places_url_hashindex + Index moz_places_hostindex + Index moz_places_visitcount + Index moz_places_frecencyindex + Index moz_places_lastvisitdateindex + Index moz_places_guid_uniqueindex + Index moz_places_originidindex + Index moz_historyvisits_placedateindex + Index moz_historyvisits_fromindex + Index moz_historyvisits_dateindex + Index moz_bookmarks_itemindex + Index moz_bookmarks_parentindex + Index moz_bookmarks_itemlastmodifiedindex + Index moz_bookmarks_dateaddedindex + Index moz_bookmarks_guid_uniqueindex + Index moz_keywords_placepostdata_uniqueindex + Index moz_annos_placeattributeindex + Index moz_items_annos_itemattributeindex > Task: _refreshUI

What code do you have in userChrome.css as that is still the most likely cause ? You can try Restore Defaults on the customize page. You can try to open a new window. You can clear the startup caches via the "Help -> Troubleshooting Information" (about:support) page to see if that has effect.

Thanks for the answer.

My userChrome.css contains:

• changing color of currently selected tab
• changing "flexible space" into a vertical separator
• hiding folder icons on bookmarks toolbar
• changing favicon of Google Maps on bookmarks toolbar

But I removed the file altogether and it didn't help. I also disabled toolkit.legacyUserProfileCustomizations.stylesheets in about:config, didn't help either.

Restore Defaults didn't help.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Launching in Safe Mode didn't help.

I use the Default theme.

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar.

Just to update: the issue is gone for now. It seems to have fixed itself with one of the latest Firefox updates. Thank you for your help.