Firefox Download Tool is a free tool which makes it simple to find and download the latest or an older version of Firefox.
Going back to a previous version isn't normally a good idea as you'll probably be removing some security patches, but could still be useful for developers.
The program doesn't need installation, and is very easy to use. You can opt to download the latest build or choose any of the last 30 versions; you're able to choose any supported language, and select either the 32 or 64-bit editions.
When you're happy, clicking Start displays a "Save As" dialog where you can choose the destination file and folder, and the program then downloads the setup file.
If you're uneasy about allowing third-party tools to download major applications, we are too, but the developer goes to unusual lengths to reassure. Once the download is complete it can provide MD5, SHA1 and SHA256 hashes for checking the download, as well as checking it directly with VirusTotal.