If the "Save visible" and "Save full page" buttons are clicked multiple times, a new tab is opened for each click

[SoftVision-CosminMuntean]

[Affected versions]:

• Nightly 49.0.1
• Mozilla Page Shot 0.1.201610172213

[Affected Platforms]:

• All Windows
• All Mac

[Prerequisites]:

• Latest Page Shot add-on (v0.1.201610172213) is installed on a clean profile.

[Steps to reproduce]:
1 Open the browser with the profile from prerequisite and navigate to any web page.
2. Click on the "Page Shot" button from browser toolbar.
3. Click multiple times on the "Save visible" or "Save full page" buttons.
4. Navigate through the newly opened tabs.

[Expected result]:

• Only one tab is opened even if the button is clicked multiple times.

[Actual result]:

• A new tab is opened for each click and on some tabs the captured image is broken.

[Additional Notes]:

• Added a screen recording with the issue: https://goo.gl/3kijS6.

[SoftVision-CosminMuntean]

It seems that this issue is reproducible on the latest Firefox Screenshots (5.2.3356) version for the "Save visible" and "Save full page" buttons.
Here is a screen recording with the issue: https://goo.gl/vGB7oF.