There seems to be a certain amount of confusion on this thread, so just to be sure that it's clear, Selenium itself will continue to work with future releases of Firefox. This means that existing WebDriver-based browser automation remains compatible with with Firefox 55+. SauceLabs', and other testing-infrastructure-as-a-service providers' support for Firefox will be unchanged. The only thing that will stop working is the Selenium IDE which is a XUL extension that allows writing selenium tests without writing any code. This is undoubtedly useful to some people, but as the original post says, there are alternatives in development that are targetting a similar niche, so the situation is not as dire as the title implies (I imagine the title is worded this way to reduce the number of duplicate bug reports that the Selenium owners have to wade through on the subject of Firefox 55+ compat. for IDE).
To expand further on Mozilla's ongoing commitment to WebDriver, we employ one of the editors of the WebDriver specification [1], are making significant contributions to the WebDriver testsuite, and are actively working on our geckodriver implementation to ensure that we have a fully-featured standards-complaint implementation as soon as possible. I also know that other vendors are working on improving the spec compatibility of their implementations, so I think the future of WebDriver is very promising, with fewer differences between drivers that aren't the result of fundamental differences in the browsers under test.