We’re running on fumes technologically as well. Besides various layout and DOM features we don’t support well like CSS grid, there are large JavaScript updates we’ll increasingly need which are formidably complex tasks. The biggest is async and await support which landed in Firefox 52, and which many sites now expect to run at all. However, at the time it required substantial changes to both JavaScript and the runtime environment and had lots of regressions and bugs to pick up. We have some minimal syntactic support for the feature but it covers only the simplest of use cases incompletely. There are also front end changes required to deal with certain minifiers (more about this in a moment) but they can all be traced back to a monstrous 2.5MB commit which is impossible to split up piecemeal. We could try to port 52ESR as a whole, but we would potentially suffer some significant regressions in the process, and because there is no Rust support for 32-bit PowerPC on OS X we couldn’t build anything past Firefox 54 anyway. All it does is just get us that much closer to an impenetrable dead end. It pains me to say so, but it’s just not worth it, especially if I, the browser’s only official beneficiary, am rarely using it personally these days. It’s best to hang it up here while the browser still works for most practical purposes and people can figure out their next move, rather than vainly struggling on with token changes until the core is totally useless.

source: HN