Something is definitely wrong on your Firefox, but I don't know what is causing it. I don't read every thread here, but I can't recall ever hearing of similar problems in the past. (Starting up with an open sidebar was caused by shutting down with an open sidebar in past threads.)

Did this just start for you in Firefox 63?

Bug reporting:

• https://developer.mozilla.org/docs/Mozilla/QA/Bug_writing_guidelines
• https://bugzilla.mozilla.org/

No. It started in the first version of Firefox that introduced the sidebar and its icon.

Is there some way I can delete Firefox, reinstall a fresh version, but not lose all my bookmarks?

ErgonomicNaturalKeyboard said

No. It started in the first version of Firefox that introduced the sidebar and its icon.

That icon has probably been there since at least Firefox 29, although its appearance changed in Firefox 57.

Is there some way I can delete Firefox, reinstall a fresh version, but not lose all my bookmarks?

Hmm, sort of. You could:

(1) Try the Refresh feature, designed to retain bookmarks and passwords while clearing add-ons and a lot of settings. See: Refresh Firefox - reset add-ons and settings.

(2) Create a new profile, which is a clean slate as far as settings are concerned. You then could restore a bookmark backup to migrate your bookmarks. (Steps below)

(3) Replace just the program files, to rule out corrupt/mismatched files. No changes to your settings. (Steps below)

New Profile

This is about a 10 minute project:

(1) Make a bookmark backup file easily accessible

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Double-click into the bookmarkbackups folder. You should see several files with the date included in the file name. Copy one or two of these to a more convenient location such as your Desktop. For example, right-click a backup file and choose Copy. Then click over to the Desktop, right-click > Paste.

(2) Create the new profile

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the Create a New Profile button, then click Next. Assign a name like Oct2018, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Set as default profile button below that profile, then scroll back up and click the Restart normally button. (There are some other buttons, but please ignore them.)

Firefox should exit and then start up using the new profile, which will just look brand new. Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can get a clean test.

(3) Restore bookmarks to the new profile

Following the steps in the following article, you'll see an "Choose File..." option you can use to point Firefox to the Desktop and access the backup file you pasted there.

Restore bookmarks from backup or move them to another computer

(4) Test

Does the sidebar behave better in the new profile?

(5) Next Steps

If all is well in the new profile, you can move forward with it. To switch back, open the about:profiles page in the new profile and switch the default profile there.

Clean Reinstall

This takes about 5 minutes. It's not about your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. It's not essential to uninstall Firefox, but you can if you like, saying No to any request about removing personal data.

It only takes a few minutes.

(1) Download a fresh installer for Firefox to a convenient location:

https://www.mozilla.org/firefox/all/

(2) Exit out of Firefox (if applicable).

If you use Microsoft Office, please change your default browser to Internet Explorer temporarily before the next step.

(3) Using Windows Explorer/My Computer (hold down the Windows key and press E to launch it), right-click > rename the program folder as follows (you might have one or both):

C:\Program Files (x86)\Mozilla Firefox =to=> C:\Program Files (x86)\OldFirefox

C:\Program Files\Mozilla Firefox =to=> C:\Program Files\OldFirefox

(4) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Any improvement?

PROBLEM SOLVED.

Solution #3 above didn't work. I didn't try Solution #2. But Solution #1, although it took a while, worked.

I took a picture of all my Add-ons so I could recreate them.

I refreshed Firefox using the button on the help page for that.

I reset all my options, reinstalled all my Add-ons, rearranged my icons in the toolbar with the URL box, and the 2 problems are gone.

If I open the sidebar, close it, close Firefox, and reopen it, the sidebar remains closed. Hurray.

If I remove the sidebar icon from the bar with the URL box, whether by right clicking on it and selecting Remove or using Customize, it stays gone. Hurray.

Additionally, I can now rearrange the icons on that bar, and they stay where I put them. Hurray.

So, although there was nothing wrong with my extensions (since Safe Mode didn't solve any of the problems), something that Refresh Firefox did fixed whatever bug had crept into my system.

Many thanks to jscher2000 for all the help.