Install User Agent Switcher Add-on for Firefox on your laptop and you'll be able to tether your laptop without getting the metro message. Metro detects when your browsing with a windows computer, but you can impersonate any device with the user agent switcher add on.
Here's the Link: https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/
Here is a link to download the user agent list: useragentswitcher.xml
Download the list. Once you install the add on, go into user agent switcher and import the list file above. Then choose the agent you want your browser to impersonate from the list. If you switch to android user agent then metro thinks your surfing with your mobile device and won't give the Metro warning message.
Here's a link
for more info.