To avoid this you need to add the line:
RewriteCond %{HTTP_HOST} !^dev\. [NC]
So the full directive is:
RewriteCond %{HTTP_HOST} !^dev\. [NC]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
First line checks if this is NOT the subdomain 'dev'. The next line checks if it is NOT the subdomain 'www'. Finally, if both conditions are true, redirect to 'www'. So if the subdomain is 'dev' or 'www' this rule is ignored, saving you from commenting it out all the time. That is why I wanted to stop using http://localhost:8888/websites/example.com, because you have to comment that rule out for local.
Be sure to clear your cache after adding that line to the www directive in your htaccess file. This works on all of my builds. My assumption now as to why I am getting the 403 with the builds from my team members is that they used a prepackaged version of ProcessWire for use with Foundation. I use the ProcessWire install that is provided by ProcessWire.
If I get any new information about my 403 issue, I will reply with what I discover.