If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

this could be a problem with the places.sqlite and favicons.sqlite files in the Firefox profile folder.

• use the "Places Database" -> "Verify Integrity" button on the "Help -> More Troubleshooting Information" (about:support) page

If errors are reported with "Verify Integrity", close and restart Firefox or reboot and retry.

If "Verify Integrity" cannot repair places.sqlite, rename/remove all places.sqlite and favicons.sqlite files in the Firefox profile folder with Firefox closed. Firefox will rebuild places.sqlite and restore the bookmarks from the most recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created
• you lose the history when a new places.sqlite is created

You may have to restore an older bookmarks backup in the bookmarkbackups folder via the Bookmarks Manager (Library).

• Bookmarks -> Manage Bookmarks -> Import & Backup -> Restore
• https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

See also:

• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional
• https://support.mozilla.org/en-US/kb/cant-add-change-or-save-bookmarks

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thank you, cor-el. The troubleshooter found no errors. In the end, I just did a bookmarks backup, removed all traces of the esr edition I was using, and installed the current regular release version for Windows 10. So far, so good. The bookmarks sidebar seemed to be working normally.

My troubleshooter says

> 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 25600KiB + The database has been vacuumed + Final database size is 25600KiB > Task: stats + Places.sqlite size is 25600KiB + Favicons.sqlite size is 15008KiB + pragma_user_version is 69 + 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 118083 unique pages + Table moz_origins has 3489 records + Table moz_places has 35659 records + Table moz_historyvisits has 67401 records + Table moz_inputhistory has 6 records + Table moz_bookmarks has 3169 records + Table moz_bookmarks_deleted has 0 records + Table moz_keywords has 2 records + Table sqlite_sequence has 1 records + Table moz_anno_attributes has 3 records + Table moz_annos has 395 records + Table moz_items_annos has 0 records + Table moz_meta has 6 records + Table sqlite_stat1 has 19 records + Table moz_places_metadata has 0 records + Table moz_places_metadata_search_queries has 0 records + Table moz_places_metadata_snapshots has 0 records + Table moz_places_metadata_snapshots_extra has 0 records + Table moz_places_metadata_snapshots_groups has 0 records + Table moz_places_metadata_groups_to_snapshots has 0 records + Table moz_session_metadata has 0 records + Table moz_session_to_places has 0 records + Table moz_previews_tombstones has 0 records + Pages sequentiality: 49.4 + sqlite_schema: 32KiB (0.1%), 30.4% eff. + moz_inputhistory: 32KiB (0.1%), 0.5% eff. + sqlite_autoindex_moz_inputhistory_1: 32KiB (0.1%), 0.4% eff. + moz_bookmarks_deleted: 32KiB (0.1%), 0% eff. + sqlite_autoindex_moz_bookmarks_deleted_1: 32KiB (0.1%), 0% eff. + moz_keywords: 32KiB (0.1%), 0.2% eff. + sqlite_autoindex_moz_keywords_1: 32KiB (0.1%), 0.1% eff. + sqlite_sequence: 32KiB (0.1%), 0.1% eff. + moz_anno_attributes: 32KiB (0.1%), 0.3% eff. + sqlite_autoindex_moz_anno_attributes_1: 32KiB (0.1%), 0.3% eff. + moz_items_annos: 32KiB (0.1%), 0% eff. + moz_meta: 32KiB (0.1%), 1.2% eff. + sqlite_stat1: 32KiB (0.1%), 3.2% eff. + moz_places_metadata: 32KiB (0.1%), 0% eff. + moz_places_metadata_search_queries: 32KiB (0.1%), 0% eff. + sqlite_autoindex_moz_places_metadata_search_queries_1: 32KiB (0.1%), 0% eff. + moz_places_metadata_snapshots: 32KiB (0.1%), 0% eff. + moz_places_metadata_snapshots_extra: 32KiB (0.1%), 0% eff. + moz_places_metadata_snapshots_groups: 32KiB (0.1%), 0% eff. + moz_places_metadata_groups_to_snapshots: 32KiB (0.1%), 0% eff. + moz_session_metadata: 32KiB (0.1%), 0% eff. + sqlite_autoindex_moz_session_metadata_1: 32KiB (0.1%), 0% eff. + moz_session_to_places: 32KiB (0.1%), 0% eff. + moz_previews_tombstones: 32KiB (0.1%), 0% eff. + moz_keywords_placepostdata_uniqueindex: 32KiB (0.1%), 0.1% eff. + moz_annos_placeattributeindex: 32KiB (0.1%), 14.5% eff. + moz_items_annos_itemattributeindex: 32KiB (0.1%), 0% eff. + moz_places_metadata_placecreated_uniqueindex: 32KiB (0.1%), 0% eff. + moz_places_metadata_referrerindex: 32KiB (0.1%), 0% eff. + moz_places_metadata_snapshots_pinnedindex: 32KiB (0.1%), 0% eff. + moz_places_metadata_snapshots_extra_typeindex: 32KiB (0.1%), 0% eff. + moz_annos: 96KiB (0.4%), 50.4% eff. + moz_bookmarks_itemindex: 96KiB (0.4%), 35% eff. + moz_bookmarks_parentindex: 96KiB (0.4%), 37.7% eff. + moz_bookmarks_itemlastmodifiedindex: 96KiB (0.4%), 60.5% eff. + moz_bookmarks_dateaddedindex: 96KiB (0.4%), 51.5% eff. + moz_bookmarks_guid_uniqueindex: 96KiB (0.4%), 64.4% eff. + moz_origins: 160KiB (0.7%), 78.3% eff. + sqlite_autoindex_moz_origins_1: 160KiB (0.7%), 72.3% eff. + moz_bookmarks: 320KiB (1.4%), 84.3% eff. + moz_places_visitcount: 352KiB (1.6%), 82.6% eff. + moz_places_frecencyindex: 384KiB (1.7%), 84% eff. + moz_places_originidindex: 384KiB (1.7%), 87.8% eff. + moz_places_url_hashindex: 544KiB (2.4%), 91.3% eff. + moz_places_lastvisitdateindex: 608KiB (2.7%), 89.8% eff. + moz_historyvisits_fromindex: 736KiB (3.3%), 93.5% eff. + moz_places_guid_uniqueindex: 768KiB (3.5%), 91.9% eff. + moz_places_hostindex: 960KiB (4.3%), 93.6% eff. + moz_historyvisits_dateindex: 1152KiB (5.2%), 94.8% eff. + moz_historyvisits_placedateindex: 1344KiB (6.1%), 96.7% eff. + moz_historyvisits: 1728KiB (7.8%), 98.1% eff. + moz_places: 11040KiB (49.7%), 99% eff. > Task: _refreshUI so it looks like there are no errors - I'm sane, vacuumed, and eff. But I have the same problem - can't expand or collapse bookmark folders. It's intermittent - happens about every day or two. Closing the bookmark panel and redisplaying it, or restarting Firefox seems to fix it, at least temporarily. However I should not have to do that. It should not break in the first place. I'm using v106.0.5. It says I can install an update. we'll see if that fixes it.