Construct does now require Intl.Segmenter support in the browser. Firefox only added support for this in v125, but that was released in April last year - the current version is v134. The latest Extended Support Release (ESR) is v128 released back in October, so even those sticking to ESR releases have a supported option. Therefore we deemed support good enough to rely on it in Firefox. The error message advises what to do - the browser is out of date and needs updating.
Support in Chrome was indeed added in v87, but that was released over four years ago, so I don't really know why anyone would still be running such an old version of Chrome unless their entire system is no longer supported. In general it's just not feasible for us to support old browsers forever - the web platform keeps improving and we want to make the most of that, and sometimes that means increasing the minimum requirements. With Chrome and Safari we try to make sure we're compatible with around 4 years of past releases. In this case we made an exception for Firefox and brought the minimum version up as it was the last browser to support a key feature that allowed us to make internationalization improvements, we waited long enough for the latest ESR release to support it, and these days Firefox usage is (sadly) much lower than Chrome or Safari so the impact of such a change is reduced.