This year, Firefox 128 ESR was released in July and the last ESR 115 update is September 3rd. At the end of this window, security exploits present in ESR 115 will not be fixed by Mozilla. This means we have a proverbial ticking clock counting down before the stable version of Tor Browser becomes out of date.
If we are unable to complete the required work before this window ends, then things get a little bit interesting. If it gets to this point, then we will continue releasing ESR 115-based Tor Browser and manually backport security-fixes from ESR 128. This is better than nothing, but as time goes on there is an increased risk that ESR 115-specific vulnerabilities will be discovered by our adversaries and go both undiscovered and unpatched by us.
Better than nothing