That address gives me an error page: https://surveys.ipsosinteractive.com/sessionControl.aspx

You are getting that prompt because the content-type is missing. Usually a .aspx file instructs Firefox as to its content type, such as "text/html" for a web page. Since it's an empty (0 bytes) file without a specified type, I suspect either there is a broken feature on a website -- do you use i-Say or have any surveys open? -- or it is caused by an add-on.

Thanks for your reply.

You wouldn't be able to access it anyway cause you'd have to be registered and they are one time sessions. In this particular case it happened to me while I was completing a survey on ipsosinteractive.com. Sometimes it happens in every page during a survey. Yeah, that's weird it's 0 bytes, isn't it? It is hard to reproduce it. If it happens again I'll try disabling the add-ons. Could it be Ublock?

It might be caused by some intermediary software (security filtering software, for example) that modified the content type header. I don't think uBlock Origin does that.

You also could try "the usual":

When you have a problem with one particular site that previously worked normally, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes. If you do not see the number going down on the page, you can reload it using Ctrl+r to check progress.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Cookies" button

In the dialog that opens, the current site should be pre-filled in the search box at the top of the dialog so you can remove that site's cookies individually.

Then try reloading the page. Does that help?

I tried cleaning cache and cookies and disabled all extensions but I still have the same issue. I only have comodo security installed but I never had issues with this before.

I was just noticing that you have a special download dialog, I think a result of using FlashGot. But the problem continues even with FlashGot disabled?

Sometimes the file that stores your download preferences becomes corrupted. You can rename the file, which causes Firefox to rebuild it from default settings. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using 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

In the first table on the page, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old.

Start Firefox back up again and your preferences for how to handle different types of content should be back at their default settings.

Any improvement?

Deleting mimeTypes.rdf seems to have done the trick. I just completed a survey from the aforementioned website and the issue did not occur. Sorry for responding with delay, but every time I have to wait until they offer me to complete a survey in order to reproduce the problem.

So, thank you very much for your help. Hopefully it is sorted. (If not I'll come back.)

All the best