URL: https://mozilla.slack.com/

Browser / Version: Firefox 58.0.2
Operating System: Windows 10
Tested Another Browser: Yes

Problem type: Design is broken
Description: Slack's "Frequently used" emoji list shows overlapping emoji in Firefox and Edge
Steps to Reproduce:
Steps to reproduce:

1. Log into a Slack instance.
2. Click the "Add reaction" smiley button in the Message text field.
3. Look at the emoji in the first column of rows 2-4 of the "Frequently used" emoji section.

Result:
There appear to be multiple overlapping emoji in Firefox 58+ and in Edge 42.17074.1002.0. See the attached screenshot.

The emoji look correct in Chrome 65.0.3325.51.

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