FF 54.0.1 - history sidebar opening - how to stop?

[dzillag]

All of a sudden in the past week, when I launch FF, the history sidebar on the left (what is launched by pressing CTRL/H) launches with it?

How do I stop this?

It's easy to "x-out-of" the history sidebar, but I did not knowingly change anything so the sidebar launches, and I'd like to reconfigure the browser to not show the history sidebar.

[dzillag]

Despite trying to find the answer before posting, I somehow missed this, which fixed it: https://support.mozilla.org/en-US/questions/1054846

Pasting the solution contents here:

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Since opening the sidebar is not the default behavior, perhaps you could eliminate the problem by removing a settings file which stores window positions and other user interface data. If you want to try that, here's how you do it:

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

"3-bar" menu button > "?" button > 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, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

"3-bar" menu button > "power" button
(menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename xulstore.json to something like xulstore.old. If you see a file named localstore.rdf, rename that one to localstore.old.

Start Firefox back up again. Does the sidebar stay closed?
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Doing this fixed the problem.