Firefox ESR 140 receives its last point release, 140.17, alongside Firefox 157 on September 29, 2026. After that the branch is done, and the institutions that pin Extended Support Release move to ESR 153. If you build for nonprofits, higher education, government or libraries, ESR is your real Firefox support floor, whatever your browserslist config says. That floor is about to move forward thirteen versions in one step, which is the largest single day change to a Firefox support matrix in years.