While add-ons are supported , they consume significant RAM on older devices.
Once installed, the legacy Firefox experience on Android 4.2.2 is a mixed bag. On the positive side, older versions of Firefox were renowned for their extensibility. Unlike the modern, stripped-down Firefox, the legacy version supports "Legacy Extensions." This allows users to install powerful add-ons like uBlock Origin. This is a critical advantage. An older device has limited RAM and processing power; modern web pages are often choked with JavaScript and heavy advertisements. By installing an ad-blocker, the user can strip away the bloat, allowing the aging processor to render the core content of the page relatively quickly.
If you are looking for the APK or documentation for research: Official Archive: You can find every historical version on the Mozilla FTP Server Community Fixes: Sites like XDA Developers