Hi, Firefox 68 will be the next ESR release series. With the release of Firefox 68.2 on October 22nd, support for ESR 60 will cease.
ESR 68 will require an updated Rust/Cargo toolchain and build dependencies not present in Stretch (nodejs 8, llvm-toolchain-7, cbindgen and maybe more). Stretch was already updated wrt Rust/Cargo for ESR 60, so there's at least no requirement to bootstrap stage0 builds this time. If we want to continue to have Firefox/Thunderbird supported in oldstable-security after October, someone needs to step up to take care of backports to a Stretch point release before October 22nd (or in case of poor timing, we can also release build dependency updates via stretch-security). Cheers, Moritz