As a workaround, does it work to save the files first and then open them after saving?

Sometimes when Firefox has problems handing off a download to another program it is caused by a corrupted settings file. That file is hard to edit, so usually we suggest renaming the file to see whether that resolves the issue. 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.

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 test your Gmail attachments. Any improvement?

Note: This resets some preferences to their defaults, such as which PDF viewer Firefox uses. If you need to update that again, please see this article: View and edit PDF files using Firefox’s built-in viewer.

Try to redo the default browser setting and temporarily set another browser as the default browser.