hello bingseto, can you test if the same is happening when you launch firefox in safe mode once - maybe a theme or extension is interfering?!

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Ctrl + B opens and closes the Bookmarks Sidebar (left side of the browser window).

Also, there are two different Bookmarks buttons in the Firefox UI. The button that appears on the right-side of the Bookmarks Toolbar, and is labeled Bookmarks, is not the button to open the Bookmarks Sidebar. The Bookmarks Sidebar button is in the Customize Palette.
https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

Yes, the bookmarks sidebar does open on the LEFT side, but my question is how to get it to open on the LEFT side.

In the past, it did open on the left side.

The Bookmarks Menu button that Firefox adds to the right hand side of the Navigation Toolbar when the Menu Bar and the Bookmarks Toolbar are hidden opens a drop down list.
That Bookmarks Menu button has a drop marker to differentiate it from the other Bookmarks button that opens the bookmarks sidebar.
So if you do not have the bookmarks button without the drop marker to open the sidebar then you can open the customize window and drag the bookmark button with only a star on the Navigation Toolbar and possibly drag the Bookmarks Menu button back into the toolbar palette.

• View > Toolbars > Customize
• Firefox menu button > Options > Toolbar Layout