– they have unpatched critical security vulnerabilities . Use them only for:
Firefox ESR 52.0esr, See All New Features, Updates and Fixes Download Firefox 12-51 52 Esr
Downloading and using old versions of Firefox is on the modern web. These versions: Legacy Add-on Support on Firefox ESR - The Mozilla Blog – they have unpatched critical security vulnerabilities
In conclusion, the span of Firefox versions 12 through 52 ESR is not just a list of obsolete software; it is a timeline of the web’s growing pains. It captures the browser's shift from a single-process relic to a multi-process modern application. While the average user has no need for these versions, Firefox 52 ESR remains a critical tool in the digital preservationist’s kit, serving as the last bastion for the plugin-heavy web of the past. Understanding and accessing these versions allows us to bridge the gap between the modern internet and the legacy systems that built it. It captures the browser's shift from a single-process
If you only need an older version for a specific feature, consider these safer options: How to switch to FF 52 ESR? | Firefox Support Forum
Many modern websites will fail to load or will look broken on these older browsers.