Remove Firefox check and make all browsers behave the same

[rpearce]

This issue is a:

• Other

Description

We should consider taking out the InstallTrigger Firefox check and make the same functionality happen across all the browsers. It's probably the safest thing to do.

Conversation Reference:
#117 (comment)

Basically, these lines (https://github.com/rpearce/react-medium-image-zoom/blob/master/src/Zoom.js#L87-L92)

const src = img.currentSrc || img.src

// See comment at top of file for Firefox check information
const newState = isFirefox ? { tmpSrc: src } : { src }

this.setState(newState)

should be changed to

this.setState({ tmpSrc: img.currentSrc || img.src })

and the isFirefox check should be removed.

cc @kingdido999 for talking sense into me.

[hippodippo]

I am happy to work/fix this. This will be my first code contribution though.