TenFourFox "10.4Fx" was designed to be as close to Mozilla sources as possible, for as long as possible. We defined our support levels for 10.4Fx in terms of "parity," viz., how similar we are to the Mozilla repository we are based on. TenFourFox has three levels of parity, as follows:
Source parity
This meant that we still built the browser from a Mozilla repository, typically the most recent Extended Support Release (ESR), using our local changeset for compatibility only. Instead of distributing the entire source, we simply distributed our changeset and binaries; you would have cloned their repo and applied our changeset to your local repo to build.
After almost seven years, TenFourFox exited source parity with the end of support for the Firefox ESR 45 branch in June 2017.
Feature parity
This parity level indicated that we were no longer building directly from a Mozilla repository but instead from our own codebase, which was once a Mozilla-supported repo and is no longer. Dropping to feature parity implies that the most current Mozilla as written cannot be hacked to compile anymore, so since our source is now necessarily diverging, we maintain it in our own repository ourselves.
Feature parity implies that we will do our best, within technical limits, to match the feature set of the current Firefox even if we are not using the same source necessarily. This typically involves monitoring Mercurial changesets and Bugzilla bugs for patches, and adapting and/or applying them if they are deemed useful. Only useful or critical features will be so ported, to avoid turning the browser into a series of potentially unstable patches of patches. Security parity, naturally, is considered just this sort of critical patch, but other improvements to rendering and performance will also be considered. Also, feature parity does not preclude implementing our own custom features, since the code base is ours, which is a small advantage.
After over ten years, TenFourFox exited feature parity with Feature Parity Release 32 in April 2021.
Security parity
This was the final state of TenFourFox. This meant the browser reached a point of maximum technical evolution, either due to completely incompatible code or the processing limits of aging machines, and is the terminal stage of support. Although some custom features can potentially be added, the browser core essentially exists in a state of arrested development. However, non-security bugs were still fixed to the extent possible, and Mozilla security advisories were still backported such that the browser was still safe for day-to-day use even if it was no longer evolving technologically.
Official security parity builds ended on October 5, 2021. Although security updates are planned to occur to the Github source tree for an indefinite period, these updates will not occur on any guaranteed schedule (or guaranteed to occur at all), users are responsible for making their own builds, and no support is provided for anyone choosing to do so.