Handle firefox private mode

Expected page load results on latest versions of the following browsers: 

| Browser    | Normal mode | Private mode  |
| ------------- |:---:|:---:|
| Chrome     | ✓ | ✓ |
| Firefox      | ✓  | ✘ |
| Safari        | ✓  | ✘ |
| Safari mobile  | ✓  | ✘ |

This fixes #129