Adobe installers are not infected. Astromenda is a browser hijacker you allowed to install (bundled with optional installs that come preselected by default - such as a toolbar) yourself by not deselecting it during the download or installation process.
Additional to MBAM, you may wish to use additional removal tools, such as AdwCleaner, to scan the computer for additional adware that may have been installed. This 'Remove Astromenda.com virus (Easy Removal Guide)' by courtesy of Malwaretips.com may be of assistance.
Then, uninstall FP and reinstall the latest and current version (14.0.0.179 for Firefox - today) using a fresh-new installer / download.