You may already be aware of this, so apologies in advance if I’m suggesting something you already know. I use Internet Explorer and on the top far right there is a little circle/cog for “tools”. If one left clicks on this, one of the choices is zoom. One can then left click on 400%, then continue to zoom if needed. I’m not sure this will help with your mom’s being able to read posts or not. She doesn’t have to sign up on FR to read, just to post (if she has dexterity issues, posting may present other problems). I’ve not used other browsers, but they probably have zoom capabilities, too. I know that Microsoft products do have other accessibility capabilities.
http://windows.microsoft.com/en-US/internet-explorer/ie-accessibilty-options#ie=ie-11
Unfortunately, she doesn’t have the patience to navigate or try to figure things out. She does email and that’s it!
If she uses something other than IE then ctrl and + will make things bigger in increments or ctrl and - will make them smaller again.