I just tweaked my monitor settings using a couple of the websites with
"free" setup images and this has certainly improved my views of photos
from a wide variety of sources. However I'm now finding that text in
Thunderbird, Google, Facebook, Amazon, etc. is a bit pale and lacking in
contrast.

FWIW the Duck's recent Yosemite pics look good to me, prior to the tweak
the gopher and river shots would have been a bit lacking in shadow detail.

I don't believe I have any significant visual impairment apart from
presbyopia commensurate with my age (68).

Any views or suggestions, short of adding a second monitor? I'm not a
sufficiently serious user for it being worth getting a proper calibrator.