Do you mean Adblock Browser? Because we recently released Adblock Browser 2.0 which is based on Chrome whereas the previous version was based on Firefox. Presumably, Chrome doesn't show Reader Mode as often as Firefox did.
There are some articles out there, however, that describe how you can enable it (see Android Authority article
) but it sounds like they're about to remove it again (see Android Policy article
I'll make sure to let the Adblock Browser team know about it as this may be a useful feature to keep and/or promote since it also blocks ads as well as other annoyances.