The Add-ons manager has different dimensions and a different location when opened during the installation of an extension than when it's opened through Tools -> Add-ons. The issue seems to come from the fact the the URL query value (location.search) is kept in localstore.rdf, which thus distinguishes between extensions.xul and extensions.xul?view=installs. A possible solution would be to use the location hash instead (extensions.xul#installs) or to pass the view variable as an additional parameter (as done for Firefox' Options dialog).
This should be fixed when the patch in bug 342098 lands.
This is fixed by the patch in bug 342098 which has just landed on trunk... resolving -> fixed
Fixed on MOZILLA_1_8_BRANCH by the checkin for bug 342098