TIP: If you are a Google Chrome user, you can also find old versions of Chrome.

Luckily, Firefox (unlike Chrome) provides an archive of its older versions and makes them available to its users on its FTP directory, which I will be mentioning below. Even if it's not ideal in terms of missing up-to-date features, the possibility of not displaying some websites correctly and potential security risks, if you definitely have to use a certain older version and you are aware of the disadvantages and potential risks, then you should find and install the version you need to use. In such cases, using an older version of your browser is kind of a must. These reasons may range from having problems with a recent update to addon incompatibility with a certain version, testing web layouts and interface functionality and support to certain web applications that you are using. However, it is also important for many of us (especially web designers & developers) to be able to use an older version of the same browser, for a number of reasons. It is good to know that dozens (even hundreds) of professionals are working really hard to keep the browsers we are using up to date, fully functional and safe. Web browser companies do a great job at constantly keeping their browsers up to date, answering the needs of users in this quickly-evolving Internet and Mobile focused world and reducing security risks of all kinds.

Older versions of firefox