Yes, it's annoyingly uninformative, isn't it. As far as I know, it's actually a revised version of the old unresponsive script warning -- without the address of the problem script.

We have a general support article addressing potential solutions to this issue: Warning Unresponsive script - What it means and how to fix it.

If you want to see the old-style dialog, you would have to disable the Multiprocess feature.

Multiprocess. Recent versions of Firefox are migrating more users to a "multiprocess" architecture where the user interface and the web content are in separate processes to improve stability. Could you investigate:

Are you using Multiprocess (e10s)?

Multiprocess creates a second firefox.exe or plugin-container.exe process to isolate the web content from the browser UI. You can check whether you have this feature turned on as follows. Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, check the row for "Multiprocess Windows" and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s.

If you are using e10s:

To help evaluate whether that feature is causing problems, you could turn it off as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autos and pause while the list is filtered

(3) Double-click the browser.tabs.remote.autostart.2 preference to switch the value from true to false

Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart

At your next Firefox startup, it should run in the traditional way. Any improvement?

Your Firefox indicated three different version of the Flash plugin (next to your question, Question Details > More System Details):

• Shockwave Flash 11.3 r300
• Shockwave Flash 22.0 r0
• Shockwave Flash 24.0 r0

The current release actually is Flash 25, so all of those are out-of-date and potential security problems. To clean out your current Flash plugins and get the latest one, you can use the steps in this article: Flash Plugin - Keep it up to date and troubleshoot problems. It has links to Adobe's uninstaller to clean up the obsolete versions, and to the install page, to get the latest.