Problem install/update add-ons. https://addons.mozilla.org/firefox/downloads/latest/7560/addon-7560-latest.xpi?src=api p.g.a.: Ogiltig filhash (möjligen en skadad filhämtning) -261

If Firefox gave you the following error message: Firefox could not install the file at https://addons.mozilla.org/******.xpi because:
Invalid file hash (possible download corruption) -261

This error message may be caused when Firefox is set disallow third party cookies. To accept the third party cookies: go to Tools -> Options -> Privacy -> tick 'Accept third party cookies'.

Thanks for answer. I am running Firefox on Mac (apple). How do I accept cookies from "third party" in Firefox for Mac?

Menu differences: Windows: Tools > Options - Mac: Firefox > Preferences - Linux: Edit > Preferences

See Unable to install add-ons (extensions or themes)
You need to enable third-party cookies temporarily.