Hi, I don't have personal experience of this add-on, but this is the nearest I can find for what you want -

https://addons.mozilla.org/en-US/firefox/addon/all-in-one-sidebar/

Features for above add-on, in particular: 'Customize the behavior of the sidebar at startup, in full screen mode and more. E.g. you can choose a sidebar panel which should open automatically at browser startup' -

http://firefox.exxile.net/aios/features.php

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

If you close Firefox with the sidebar open then Firefox should reopen automatically with the sidebar open.

Scribe said

Hi, I don't have personal experience of this add-on, but this is the nearest I can find for what you want - https://addons.mozilla.org/en-US/firefox/addon/all-in-one-sidebar/ Features for above add-on, in particular: 'Customize the behavior of the sidebar at startup, in full screen mode and more. E.g. you can choose a sidebar panel which should open automatically at browser startup' - http://firefox.exxile.net/aios/features.php If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

cor-el said

If you close Firefox with the sidebar open then Firefox should reopen automatically with the sidebar open.

No, I try that, but not working.

Try to rename/remove the xulstore.json file in the Firefox profile folder.

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

If you use cleanup software like CCleaner then check the settings for the Firefox application.