Firefox uses data from Google's Safe Browsing to check whether files from a website are safe to download.

You can set the browser.safebrowsing.appRepURL pref to an empty string value if you are willing to accept the risks that come with disabling this security feature.

Bypass Firefox's Blocked: May contain a virus or spyware message - gHacks Tech News:

• http://www.ghacks.net/2014/11/19/bypass-firefoxs-blocked-may-contain-a-virus-or-spyware-message/

• [/questions/1050944] How can be this virus or spyware?

Ich verstehe nicht ganz: Muss ich befürchten, dass ich nun eine Schadsoftware auf dem PC habe?

Not necessarily. Firefox uses a special Google service to check for malware and Google has extended this service to include websites that host software that is considered as potential harmful. The presence of one such software that is considered as harmful is sufficient to flag all software hosted on that domain and it is up to you to decide how to proceed (do not use this software or ignore the warning or try to find another source where to download it).

• https://support.mozilla.org/kb/how-does-phishing-and-malware-protection-work
• https://www.google.com/about/company/unwanted-software-policy.html