For the curious:

Content-Disposition: What are the differences between "inline" and "attachment"? – Stack Overflow
https://stackoverflow.com/q/1395151/38108

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With Waterfox Classic, I switched from legacy 'InlineDisposition 2' to this extension (from the same developer) following an issue at an Adobe site. The issue is not reproducible with this extension.

Great add-on, but can we get a filetype whitelist as well? I only want this enabled for images and PDFs, but it also affects video downloads. This is particularly problematic on file hosts with download limits -- the file loads in the browser, and when you try to save it, you get nothing because you've reached your download limit and have to wait several hours to try again.

EDIT: Thanks for the response, but this doesn't solve my issue. Still very interested in a filetype whitelist.

Need a support link so I don't have to report temporary issues here...
When this addon is off I have no issues, turn it back on and I experience the following:

Whether attempting to sign in to change password via gmail or google search page, or attempt to open gmail contacts I get a dialog... such as:
"Opening ServiceLogin"
ServiceLogin
which is: application/binary
from: https://accounts.google.com
What should Firefox do with this file?
(radio button) open with (choose button)
Save File
Do this automatically for files like this from now on.

Again, when I turn this addon off I have no problems signing in / don't get the dialog box. Enable this addon, the problem immediately returns.