Sorting it out with the technician obliged me to refresh Mozilla

Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

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

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

https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update

Hi FredMcD and thanks for your swift respnse and ideas. No, I don't see anything on the desktop called Olf Firebox. However I have sight of the Bookmaks backup file, where I see some 15 entries, identical in name and size, except for creation date, the last being yesterday, sometime shortly after I first fired up the PC and, with it, Mozilla. However, I have failed miserably to load this back up. I have equally failed both to re-install the command line symbol (3 forward slashes ?) and also failed with anything which enables me to show all stored bookmarks at left of screen (PF11 toggling did not produce any reseult). So please, tell me where I'm going wrong/missing out? Thanks again and very best; PeterT

. . . . I have sight of the Bookmaks backup file

That should be a folder.

Restore Bookmark Backups

Firefox automatically makes backups of the bookmarks. These are stored in the bookmarkbackups folder.

Open the Bookmarks Manager; Press the Alt or F10 key to bring up the toolbar, and select Bookmarks. Hot key is <Control> (Mac=<Command>) <Shift> B. Firefox 84+ <Control> (Mac=<Command>) <Shift> O.

Once the window is open, at the top of the page, press the button labeled Import and Backup. Select Restore. From the list, pick the file you want and click it.

The backups are stored in a jsonlz4 file named something like this; bookmarks-2016-06-29_1424_enD1GTH ya0JmicnYWg4V4g==.jsonlz4

In this example; 2016-06-29 = date created, 1424 = number of entries.

https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks

https://support.mozilla.org/kb/forum-response-recover-bookmarks-after-update

Thanks again. I have duly gone through the steps of locating and selecting the latest (created 2/2) of the jsoniz24 files present on the destop in the target folder (they all contain 386 or 387 entries). I have gone through the Bookmanagers steps you showed me. And in the process I have managed to put back on the command line the Library symbol. However, when I click on that it reveals only a list of the "Other Bookmaks" part of the old complete set. The items still not in evidence are the key, regularly-consulted bookmarks, organised into folders - although bizarrely the "Other Bookmarks" number far less than 386. Also, I still see no sign of how to turn back on the command which enables the Bookmaks to be shown at left of screen. Your further advice will be much appreciated. Thank you and best wishes, PeterT.

. . . the command which enables the Bookmaks to be shown at left of screen.

Do you mean the Bookmarks Sidebar? <Control> B

. . . it reveals only a list of the "Other Bookmaks" part of the old complete set

Have you tried selecting different folders in the folder 'tree' on the left?

Hello FredMcD I have just posted a "Solved!" message but I wanted to be sure to express my thanks to you for your rapid and helpful replies to me. Your last piece (about <Control> B did the trick , enabling me to see and re-open principal bookmarks, all now safely visible left-of-screen and allowing us (machine shared with my wife) to navigate as before. Best wishes, Peter T

Glad to help. Safe Surfing.