I have the issue as well.
I remedied it by setting the command in properties to:
gksudo synaptic
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https://bugs.launchpad.net/bugs/827878
Title:
Synaptic can no longer be started from
Public bug reported:
Binary package hint: firefox
Lucid
Firefox 3.6.3+nobinonly-0ubuntu4
When several(4-5) tabs are opened, closing one sometimes results in the tabs
getting jumbled, that is the page that is shown when a tab is clicked on is not
the right page. Sometimes it is the closed page
** Attachment added: Dependencies.txt
http://launchpadlibrarian.net/49074193/Dependencies.txt
** Attachment added: ExtensionSummary.txt
http://launchpadlibrarian.net/49074194/ExtensionSummary.txt
** Attachment added: default_profile_pluginreg.dat.txt