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

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

• Help -> 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

Hi cor-el,

Thanks for the suggestion, which I tried. But unfortunately renaming xulstore.json did not resolve the problem of keeping the sidebar width from changing when Firefox is closed and subsequently opened, say after rebooting.

Again, thanks,

David

What do you currently open in the sidebar ?
Is this a builtin page like bookmarks or history or Sync (Tabs) or something from an extension ?

This data is stored in xulstore.json when I open the history sidebar then I see this stored in xulstore.json:

"sidebar-box":{"sidebarcommand":"viewHistorySidebar","checked":"true","width":""},"sidebar-title":{"value":"History"}

Hi cor-el,

Believe it or not, I discovered the parameters you cited, above, were absent in the xulstore.json when I compared the xulstore.json on my desktop computer (the one with the problem) with the xulstore.json file in the Firefox installation on my laptop. Why those parameters were missing on my desktop computer seems to me to be a mystery, but in any event I decided to roll the dice by copying the xulstore.json from my laptop and inserting on my desktop (of course, I kept a backup of the xulstore.json just in case). And somewhat surprisingly my problem was solved and so far I don't appear to have introduced any new problems.

But do you have any idea why the sidebar-box parameters would have been absent from the original xulstore on my desktop machine? In fact, even after I renamed xulstore and restarted Firefox, the sidebar-box parameters were again not present.

Thanks for your help, as you definitely lead me in the right direction, which I otherwise would never have gotten to.

Best,

David