In case you are using "Clear history when Firefox closes":

• do not clear the Browsing History

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

Make sure that you do not run Firefox in (permanent) Private Browsing mode (Never Remember History; privénavigatiemodus).

• https://support.mozilla.org/kb/Private+Browsing

• Firefox > Preferences > Privacy > Firefox zal: "Aangepaste instellingen gebruiken voor geschiedenis"
• Deselect: [ ] "Altijd de privénavigatiemodus gebruiken"
• https://support.mozilla.org/kb/Options+window+-+Privacy+panel

Thank you for your quick reply! But: My Preferences/Privacy has always been set on "Remember history"(Geschiedenis onthouden) This is my best option after having problems before (question must be somewhere in the archives here) -I can't use, and I don't use "Use custom settings for history", besides that "always use private navigation modus"("Altijd de privénavigatiemodus gebruiken") has been already permanent deselected. -Till now everything went ok: history appeared automatically in Sidebar. -but now, like I mentioned before, I first have to choose first one of the options in "Show" in the sidebar-window. -Only than the history appears

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can check for problems caused by a corrupted localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Thanks again. I tried everything you advised, without success.... Except "You can check for problems caused by a corrupted localstore.rdf file." I will try that tomorrow. Perhaps by replacing that whole file by the same file from Time Machine. Might that be a good idea?

I checked everything according your advise. No success.... I also replaced from Time Machine the complete Profiles folder, the profile.ini, and the org.mozilla.firefox.plist. No success....

Still no relevant solution.......

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Thank you very, very much for taking the effort to reply. But I'm sorry, I'm rather afraid…..to do these actions…I'm not in a great hurry, so take your time! Could you please be so kind to tell me step, by step, the right order of were to start etc. in the given link:

Using the profile manager to make Firefox-profiles and to remove

Inhoudsopgave (Content)

-De profielbeheerder starten (to start the profile manager)

-Een profiel aanmaken (to creatie a profiel)

-Een profiel verwijderen (to remover a profiel)

-Een profiel hernoemen (to rename a profiel)

Opties (Options)

-Offline werken (working offline)

-Niet vragen bij het opstarten (no asking at starting)

-Een profiel verplaatsen (to move a profiel)

-Gegevens uit een oud profiel terugzetten ( to replace data of an old profiel)

I would be very grateful for your eventual answer!

EDIT: A FURTHER AND PROPER look on your links, shows that the STEPS ARE THERE already. So I'l find it out myself, and give you the result of my Operation, independant of success or not!

I think)I hope) that I found the solution, although not perfect yet: I deleted from the profile-folder in a first step these extensions. Have to do more:

-extensions.ini

-pluginreg.dat

The sidebar/history shows after two times restarting Firefox "Vandaag"("Today") again