Replace process.hrtime with a monotonic counter

Per issue #336, process.hrtime does not seem to be guaranteed monotonic,
so replace that strategy with our own monotonic counter implementation.

Also, fix a few inconsistent tests, as well as adding support in the
test suite for pinning to older versions of Firefox (set
``conf.TESTING_FIREFOX_BIN`` to the firefox binary to use), to tolerate
lag between new Firefox releases and support from selenium-webdriver,
e.g.
https://stackoverflow.com/questions/23412912/selenium-send-keys-doesnt-work-if-input-type-number