Description of problem:
Myspell is obsolete, please move to hunspell. Hunspell dicts are in /usr/share/hunspell and they are compatible with Myspell.

Possible (not tested) patch:

Index: SPECS/firefox.spec
===================================================================
--- SPECS/firefox.spec	(wersja 206356)
+++ SPECS/firefox.spec	(kopia robocza)
@@ -34,7 +34,7 @@
 %if 0%{?prel}
 %define release %mkrel -c %prel 1
 %else
-%define release %mkrel 2
+%define release %mkrel 3
 %endif
 
 Summary:	Next generation web browser
@@ -125,7 +125,7 @@
 Requires:       xdg-utils
 # fixes bug #42096
 Requires:	mailcap
-Suggests:	myspell-en_US
+Suggests:	hunspell-en
 Suggests:	%{_lib}canberra0
 Suggests:	%{_lib}cups2
 Obsoletes:	firefox-ext-weave-sync
@@ -334,7 +334,7 @@
 
 # use the system myspell dictionaries
 rm -fr %{buildroot}%{mozillalibdir}/dictionaries
-ln -s %{_datadir}/dict/mozilla %{buildroot}%{mozillalibdir}/dictionaries
+ln -s %{_datadir}/hunspell %{buildroot}%{mozillalibdir}/dictionaries
 
 # (lm) touch and %ghost bookmarks.html to a proper uninstall
 touch %{buildroot}%{mozillalibdir}/defaults/profile/bookmarks.htm