Change browser.download.open_pdf_attachments_inline to true in about:config.

How does the server sends those PDF files if you check this in the in the Network Monitor?

• https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/

@cor-el Here are 3 screenshots of the Network Monitor regarding an example of a pdf that was (unfortunately) automatically saved.

Hello

Other information What information is stored in my profile? https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_what-information-is-stored-in-my-profile Download actions: handlers.json This file stores your preferences that tell Firefox what to do when it comes across a particular type of file. For example, these are the settings that tell Firefox to open a PDF file with Acrobat Reader when you click on it. For more information, see Manage file types and download actions in Firefox.

Try

Go to your profile https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_how-do-i-find-my-profile Open Folder Then shut down Firefox Then, in your profile, delete the file handlers.json or rename the file (for example) handlers.json-old Next, start Firefox

Then Perform your checks https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file

handlers.json will be recreated automatically For information purposes You can view the contents of the file handlers.json (e.g. with Notepad)

Firefox always needs to download and save the file to be able to display the file in a tab.

You can set browser.download.start_downloads_in_tmp_dir => true on the about:config page to make Firefox use the OS temp folder for downloading and opening files.

You can open the about:config page via the location/address bar. On the warning page, click "Accept the Risk and Continue" to open about:config.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox