[EuiDataGrid] Footer row jumps in Firefox

[ryankeairns]

This Kibana PR (with link to related issue) provides a near-term fix until this can be addressed at the EUI level. The issue has to do with the summary row being slightly out of position upon initial load and then 'jumping' into place upon hover.

From the originating issue:

Screenshots (if relevant):

[ryankeairns]

Adding image from 2nd case observed in Discover (on FF):

[timroes]

Fixed via #4869