If you close (File -> Exit) Firefox with the sidebar visible, then Firefox should reopen the sidebar automatically. Otherwise try to rename/remove xulstore.json in the Firefox profile folder with Firefox closed.

You can use the button in "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Renaming / removing xulstore.json in the Firefox profile folder does not clear the issue.

Firefox will only show the Bookmarks toolbar if the last closed window has the sidebar open. Does this work if you only open one Firefox window and open the sidebar in this window and close and restart Firefox?

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

'Firefox will only show the Bookmarks toolbar if the last closed window has the sidebar open. Does this work if you only open one Firefox window and open the sidebar in this window and close and restart Firefox? NO

If your problem persists in Troubleshoot Mode, it is not being caused by an extension, theme or hardware acceleration. NO Attachment FF#02

If the problem is no longer happening, then hardware acceleration was likely the cause. You can try updating your graphics drivers to see if that fixes it or simply run without hardware acceleration. Still broke FF#03

Switch to default heme Already using it FF #01

Test for faulty extensions All extensions disabled & issue gone away FF #04

Re-enabled Dashlane extension & updated Issue returned FF #00

Disabled Dashlane extension

Re-enabled Malwarebytes Browser Guard & updated Issue Returned FF #00

How are you closing Firefox?

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

I am / was closing by clicking the close X on the Firefox.

Using

"3-bar" menu button -> Exit (Power button) OR Windows: File -> Exit

when no other windows open / multiple tabs open, does NOT clear the issue.

Having a spare moments today, I completely uninstalled Firefox v 127.0.2 & re-installed it - the issue as reported still remains.

I'm begining to think this issue is related to my Windows 11 OS, v 24H2 Build(26100.1000); it started on Build(26100.994)

I have the same problem too. I have to now always goto View...sidebar...bookmarks to have the bookmark menu appear on the left side. Makes no difference how I close Firefox or how many windows are open. When they are all close and I open firefox again I have to repeat the process to see the bookmark menu. I stayed with firefox because I like to always see the bookmark menu. I guess I dont have to stay with such an inferior browser. Firefox now officially sucks!

Raised with Mozilla Tech Support as a possible bug.

Note, you can open the bookmarks side panel with ctrl+B.

This doesn't solve the issue of course, but as a workaround it might be a quicker method than navigating through menus.

Ian, would you mind posting a link to the bug report here, for reference?

Raised with Mozilla Tech Support as a possible bug.

Not sure what you mean by Mozilla Tech Support as Mozilla does not have any form of call centres or such to do any one on one support by phone, chat or email for their Firefox web browsers products.

Perhaps you meant you filed a bug in the general issues tracker Bugzilla ?

Bug#1910255 - Everytime I load Firefox the bookmarks sidebar is not shown.

https://bugzilla.mozilla.org/show_bug.cgi?id=1910255

Since 127 no longer xulstore.json is used to store the sidebar state, but this is now stored in sessionstore.jsonlz4 as part of session data. If you clear the browsing history when you close Firefox, then the sidebar state is lost.

This is being investigated.

• 1908019 - onPurgeSessionHistory shouldn't delete UI sidebar state

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

Are you using "Clear history when Firefox closes" to clear the browsing history as that currently will close the bookmarks sidebar ?

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings