Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

I already tried that, but no success either...

When you click the attach button, does the popup window to select your file appear?

If not, it could be because Firefox is blocking popups on the Catharina Ziekenhuis domain that hosts your email. To fix this, you will need to allow Firefox to show popups on this domain.

To do this, please add an exception for the Firefox popup list for the catharinaziekenhuis.nl domain. For more information on how to add a popup exception in Firefox, please refer to this Mozilla support article:

• Pop-up blocker settings, exceptions and troubleshooting

Please let us know if this solved your problem.

Thanks again for your advice. Unfortunately, this didn't solve the problem. I get the pop up screen, I can select a file, but afterwards I get the message the file could not be attached to the mail. It doesn't matter whether the file is on my laptop or in a dropbox or onedrive directory, whether it's opened or not...

The troubleshooting system information that you have provided shows that there is a user.js file in your Firefox profile. This file has been known to modify the behavior of Firefox and it can cause issues.

Try the following:

1. Close Firefox
2. Open your Firefox profile
3. Delete the user.js file
4. Open Firefox

For more information on your Firefox profile and how to access it, please refer to this Mozilla support article:

• Profiles - Where Firefox stores your bookmarks, passwords and other user data

Let us know if this solves the issue.

Warning: To avoid data loss, it is recommended that you back up your Firefox profile prior to performing the steps listed above.

Thanks again. This almost fixed the problem, I can forward mails with attached documents now, that was not possible before. However, to compose a new mail and attach a file still doesn't work. What changed is though is that first, I immediately got the message I couldn't attach the file, and now there's this short moment where it seems to succeed (see image) but after a short while I still got the error message.

But now I got another problem, I can't reply on e-mails anymore, I get a similar message as when trying to attach a file: this e-mail can't be sent right now, try again later...

Does this still occur on Microsoft Edge/Internet Explorer?

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Try to rename/remove the mimeTypes.rdf file in the Firefox profile folder to reset all file actions.

• http://kb.mozillazine.org/mimeTypes.rdf
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

Wesley Branton said

Does this still occur on Microsoft Edge/Internet Explorer?

I renamed the mimetypes file, it seems to have helped a little bit. There are some small files that I'm able to attach, so that's a first. however, if I try to attach files that are bigger than 20 kB it still fails.

What's strange now is that if I try to attach a file, for about 5 seconds nothing seems to happen, but than the attaching of the file is started

If there is a delay then your security software may be scanning the file before passing it to the website.

Did you check that like I wrote above?

cor-el said

If there is a delay then your security software may be scanning the file before passing it to the website. Did you check that like I wrote above?

Yes I stopped al security software but the problem still exists. I was trying to look at the webmail options, then I got this error message:

Client Information

User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:46.0) Gecko/20100101 Firefox/46.0 CPU Class: undefined Platform: Win32 System Language: undefined User Language: nl-NL CookieEnabled: true

Exception Details

Date: Tue May 24 2016 15:04:17 GMT+0200 Message: SyntaxError: expected expression, got end of script Url: https://mail.catharinaziekenhuis.nl/ecp/?rfr=owa&owaparam=modurl%3D0&p=account# Line: 0

Call Stack

ErrorHandling.$EM@https://mail.catharinaziekenhuis.nl/ecp/15.0.1156.6/scripts/common.js:1:172926 ErrorHandling.showUnhandledException@https://mail.catharinaziekenhuis.nl/ecp/15.0.1156.6/scripts/common.js:1:171997

Detailed Call Stack

Does this make any sense?