Did you uninstall the Firefox 4.o beta version, or just go back to using 3.6?

If you actually uninstalled 4.0 beta, I hope you didn't select Remove my Firefox personal data and customizations in the uninstallation routine. If you used that option, you deleted all your Firefox data, for all Firefox versions.

febe, xmarks, and mozback only work for recovery if you did a backup with those programs / extensions before you uninstalled Firefox. They won't recover data that you deleted from your PC; they aren't file recovery programs. They only restore their own backups.

im trying to uninstal firefox 4.0 beta and it wont let me

You can delete the folder where the Firefox 4.0 beta is installed.
See http://kb.mozillazine.org/Installation_directory

If you created a new profile for that version then you can delete those folders as well.
See http://kb.mozillazine.org/Profile_folder_-_Firefox

This same problem happened to me as well. Why the hell would Firefox delete the bookmarks of the 3.6 version as well? When they asked if I wanted to delete the bookmarks, I thought it was only for 4.0. After all, it is a beta version. Didn't Firefox realize that a lot of people would revert back to 3.6?

I've tried to import older version of my bookmarks, but it doesn't work. Does anyone know how to re-install the old bookmarks?

I have the EXACT same issue as the OP. This frankly may be the straw that pushes me into the arms of Chrome.

IS this hope for a fix in sight or are my thousands of books marks lost for good?

Sorry, I am afraid most of this is rather like the proverbial door & bolted horse.

One thing I did think of is that, I am not sure how firefox deletes the profiles on uninstall, if they go into your recycle folder and that has not been cleared they may still be there. Look for files with file extension.JSON I doubt that is what firefox does, and I am not going to trash mine to find out, but it is worth a look.

For future reference besides the reminder not to use the remove data option note:

• recent versions do keep about 5 days auto backups of bookmarks etc which are recoverable, unless the profile etc is deleted
• it is possible to set up additional profiles , they are ideal for testing beta versions of firefox with their own separate bookmarks
• bookmarks may be exported as a means of making your own manual backups see Restore bookmarks from backup or move them to another computer

If you deleted all Firefox profile folder via the uninstaller then your only hope is to use an undelete utility to restore the JSON backups in the bookmarkbackups folder and restore the bookmarks from that file in the current Firefox profile. A JSON backup is a text file and much smaller that places.sqlite, so your chances are higher to undelete such a file.

Recuva: http://www.piriform.com/recuva

• http://kb.mozillazine.org/Recovering_a_missing_profile
• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

You may have corrupted your your places.sqlite and/or your places.sqlite-journal files.

• Simply go to your "Help" tab in FF.
• Select "Troubleshooting Information" from the drop down menu.
• Under the "Application Basics" heading click on the "Open Containing Folder" button.
• In the file explorer window that opens up, select the "Type" header to put the files in order
• Look for any files of that type named places.sqlite, places.sqlite.corrupt, places.sqlite-journal or places.sqlite-journal.corrupt.
• Rename or delete these files.
• Close the window.
• Restart FF and it will regenerate new non-corrupt places.sqlite and places.sqlite-journal files.
• In the FF Menu Bar click on "Bookmarks".
• From the drop down menu select "Organize Bookmarks"
• In the new window select "Import and Backup" from the menu bar.
• Hover over the "Restore" button then move your cursor over the fly-out menu.
• Finally, click on the date of your last know good backup file.

Your Done!

In all cases like this I would suggest the number one priority should be to copy the existing profile files somewhere safe, preferably off the existing hdd. Then decide whether file recovery software needs to be used.

I was rather surprised to discover Firefox clears all profiles rather than just the version being uninstalled. A recent question with a happy ending is: I deleted by mistake bookmarks by uninstalling the previous version of Firefox beta 4.011b... Is there anyway to recover them? so the suggestions to try file recovery software do sometimes work, and presumably the sooner that is tried the better.

Please look at the age of a thread before answering, especially when the thread is marked as Solved by the Owner - this one is 8 months old and has been dead for two months since the last answer. There are enough current threads that need answers and older threads that haven't been answered at all, so please concentrate your efforts where it helps Firefox users the most.