some websites send certain files with an "attachment" header and firefox shows a popup-dialog to "open with X or save file", even for files that firefox could display, like *.htm or *.jpg or *.pdf or *.mp3
here is a test page: http://test.greenbytes.de/tech/tc2231/#attwithasciifilename
that behavior is desired in some cases, with large files or with actual "download links", but mostly its annoying and we just want to see the file ....
.... and find this addon to silently "rewrite" the disposition header. no more popup, just shut up and show me your bits!
Your plugin works maybe half the time. For example, this link doesn't work: (https://www.st.com/resource/en/datasheet/stusb4500.pdf). I tried this link with the "Always Preview" plugin and it worked fine.
I only wish I could selectively enable/disable it via a toolbar button and/or a configurable whitelist and blacklist of host names. Workaround: go to about:addons and disable/enable the extension, this takes effect immediately.