[BUGFIX] RTE Spellcheck dialogue opens with excessive height

By default, the RTE Spellcheck dialogue opens with excessive height
in Firefox.

Releases: master, 6.2
Resolves: #62113
Change-Id: Ifd92417ddc7c688bb752c4460a4992daf7173651
Reviewed-on: http://review.typo3.org/33216
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>