Salesforce has a great security model for 2 factor authentication and trusted systems. Sometimes, however, it can be onerous.
If you want to ensure that a user can access Salesforce without requiring a verification code or security token, you need to "whitelist" their IP on Setup => Security Controls => Network Access.
If you have a lot of IPs to whitelist, or you want to whitelist dynamic IPs, or you simply want to disregard this requirement altogether, enter this extension: Whitelist All IPs for Salesforce!
1) Enable this plugin
2) Navigate to Setup => Security Controls => Network Access in your Salesforce org (must be an Admin)
3) Click the Whitelist All IPs button
4) Wait! There is a progress indicator, it can take 30 seconds to 3 minutes depending on your connection.
Huge credit to Moti Korets for his SFDC Debug Logs extension, which had this "hidden feature"(https://github.com/motiko/sfdc-debug-logs).