Phil, seems like you certainly learned your lesson! :D
Firefox is safe if you download it from mozilla.org. The also do give the hash values under releases.mozilla.org and if you are really paranoid, you should check both MD5 and SHA values. Even though MD5 is old and not so secure algorithm nowadays, it is orders of magnitude harder to get both MD5 and SHA values to match with a modified file than just one of them. Generally just checking the SHA value is quite enough.
You can also use virustotal.com to check if the file you downloaded is ok before running or opening it. The service will calculate the hash in you browser and send it to them for checking against their library of files that they have analyzed with dozens of antivirus software. If they don't have an analysis for your file, the file is uploaded for analysis, which usually takes only minutes.
It's a very good practice to run anything you're not sure about, and any new executable you're about to run, through virustotal.