Found it.
There's an app function for that I have Chrome installed (don't -all- Android phones have it installed?) and I also have Firefox installed and set as the default browser (in Android settings.) AM has never used anything but Firefox for opening URL links.
I can easily imagine that some custom-made ROMs and rooted phones can have no Chrome installed.
Besides, even if it installed, you can
disable it.
We are talking about "opening links inside Aquamail", which became available only recently.
If Aquamail doesn't open a link withing itself, either you haven't updated to that version, or you have disabled that option. (Or you just haven't opened any links since the recent update.)
Or, maybe Chrome is disabled (Settings -> apps -> {Chrome} -> disable. One user has reported in another thread that disabling Chrome also disables opening links inside Aquamail.