Firefox showing unresponsive script warning isn’t new, but the situation when it is shown is important, you may also get it when you try to import bookmarks from an HTML file into Firefox.
Here is the warning dialog: A script on this page may be busy, or it may have stopped responding. You can stop the script now, open the Script in the debugger, or let the script continue, the script that caused this warning is this one: chrome://browser/content/places/editbookmarkOverlay.js:1112 chrome or it can be this also: chrome://browser/content/browser-places.js:1835
If clicking on Continue or Stop script for two or three times, hasn’t worked, then try the below steps.
1. Close Firefox browser.
2. Open Run dialog and open Firefox profiles folder by typing this :%appdata%\Mozilla\firefox\Profiles
3. Open the default profile, and delete Places.sqlite file
Now open Firefox and visit bookmarks manager, and try to import bookmarks from HTML file backup, it should work, if the script dialog appears, then clicking on Continue or stop the script will work, this has worked for me.