I had the very same problems on Sandboxes.
I have opened Salesforce ticket and they referred me again to this SSE post.
And also to a similar SSE post.
Jermaine's answer to that post helped for me.
If I copy the URL for the developer console into a new window, it opened in the new window without issue. After that I was able to open it directly as popup window from Salesforce.
Later Salesforce support shared this knowledge article with me.
I believe this should always help.
Knowledge Article Number: 000205278
Description: When I'm trying to open the Developer Console it freezes the browser and, depending on the browser, we can see the following error:
"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."
Resolution: The solution would be to reset the IDEWorkspace for the affected user. To do this, please:
- Open the Developer Console as the affected user.
- When the error appears, please the "Debug Script" button. If you don't have a "Debug Script" button you'll have to open the Browser's Developer Tools manually (please scroll down to the bottom of this Article for additional information).
- Move to the 'Network" tab of the browser's Dev Tools.
- You'll find a number of requests, if you don't, please refresh the browser. We're interested in a request with the following format:
- The 'Id' value is the one we are looking for. Please copy that.
- Open Workbench. Select "Utilities" and then "REST Explorer".
- Do a GET request for "/services/data/v29.0/tooling/sobjects/IDEWorkspace/1deQ00000004HmJIAU" - remember to replace the "1deQ00000004HmJIAU" parameter with the "Id" relevant to your org that you obtained in step #5.
- You will see the content of the affected user's IDEWorkspace.
- Do a DELETE request.
- Re-open Developer Console
After following these steps the Menus should be available again and the error message should no longer appear.