Does this happen with specific bookmarks or any bookmark? If you can keep track, could you post the bookmark code next time it happens?

You can right click the bookmark and then click "Properties" to pull up a dialog box with the URL data, post that next time it happens. I suspect there's some extra code in a couple of your bookmarks that's causing this to happen, if not, something else is going on.

I'll have to wait until it happens again. So far it has been completely random since the most recent time was a bookmark I only created this week.

If it happens again, I'll mark it.

It happened again, but this time it was during startup. It appeared shortly after I first opened the browser on computer startup. Steam booted up at around the same time, as well as Spotify, but I doubt it's related.

Here are the requested screenshots from before:

If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

You can check for problems with the places.sqlite and favicons.sqlite files in the Firefox profile folder.

• use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page

If errors are reported with "Verify Integrity", close and restart Firefox and retry.

If "Verify Integrity" cannot repair places.sqlite, rename/remove all places.sqlite and favicons.sqlite files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created

See also:

• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file
• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

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

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

I've done all of the above now as suggested by FredMcD and cor-el. The tab still showed up today, and there doesn't seem to be any trace of malware according to all four of the suggested apps.

Is there any situation in which Firefox fetches the default user? I created two user profiles on this computer (one for me, one for my wife) and she has never encountered this issue. However, my user profile is set as the default, and has had this issue where it pulls up the http://[username] tab from time to time.

Is it possible that because my username [David] is the default user, Firefox is fetching its profile and as a result of not finding something it's looking for, it's defaulting to searching for that webpage?

dfishbait said

I've done all of the above now

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar.

I created a new default profile and messed around for a while. Nothing popped up.

I'm starting to think there's something up with the profile itself...

Were your bookmarks and settings copied to the new profile?

No, I did not port them to the new profile.

Let's try this: With Firefox closed, Copy these files from the old profile to the new one.

logins.json and key3.db [v58+]key4.db : These are the password files. Copy Both !

places.sqlite : History and Bookmarks.

After, test run Firefox.

So far the last tip seems to have worked. I'll post again if something happens, but for now it seems to be clear.

It's hard to tell because of how infrequent it does happen.