It blocks ads just fine, but it also makes Outlook webmail display a blank page when I try to create or reply to an email. (This is the webmail interface to our company's installation of Outlook--no idea whether this also affects Outlook365.)

The problem (with some ad-ons, not this one explicitly) is described here: https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems. I disabled and re-enabled AdAvoid several times to verify that it was indeed this add-on, and not another one, nor was it hardware acceleration.