How to unmask the email addresses when viewing a topic on the web
To remove the email masking from a topic, click the three dots (...) in the middle of any masked email address in that topic. You may need to click the "More options" link to view an author's masked email address. On the page that appears, simply type the squiggly word into the box that appears below it and click "OK." You'll return to the same topic, but all the email addresses will be fully visible. Please note that you'll need to repeat this process each time you visit a topic, even if you're returning to one you've already unmasked.
Note that Google Help Groups (e.g. Gmail Discussion Group) use full email masking. As a result, the three dots will not appear, making it not possible to unmask an email address in one of these groups.