Meh, why hard-coding all the paths in a portable release. Paradox and not customizable
Have a look at Diashapes Portable (not supported anymore, works with Dia 0.97 only) and Dia Portable. User could set custom install paths, settingsdirectory, appdirectory ... and with the proper ini-entries it just works. Always.
Oh, and there is a major bug:
(line 95) {IfNot} ${FileExists} `$EXEDIR\FirefoxPortable.ini`
is always true, so the default FirefoxPortable.ini will be copied every time and overwrites a custom one. No chance to define AdditionalParameters, LocalHomepage or something else.