hi ujr, to recreate your problem, i set my pdf/handling identical to yours and went to download a pdf file.. the download dialog initiated instantly without any problems, i came to a few conclusions(and i might be wrong but try them anyways)
first don't try to set a mime type manually from the mime types options dialog, if that mime type is already registered through a plugin..i think the proper way to do it is this: (using that newly created profile)
first delete the pdf mime type from the mime dialog then go to preferences
in preferences, click on file types and select by type tab
if you find pdf listed there (you should if you have adobe's reader).. then double click on it and select'save to disk'
now go to to mime types dialog again, you should now find pdf listed automatically in there.. now double click on it and deselect ask me before saving and check if it still freezes.
you say this happens with other files also not just pdf, so i think you should first check in preferences> file handling first to see if there's a plugin to handle those file types..
if there is, then set how to handle it from there first before manually adding it from mime types(it could be a conflict between what you're adding and the plugin if available)
if a plugin doesn't exist for that type, then manually add it in mime types and it's very important before adding this type to make absolutely sure.. that you're adding it with it's right classification as how its' registered in windows registry.
to check that..do this, open regedit:
and if you ant to check the classification of pdf..they search for .pdf etc
if you followed these steps and it's still freezing up, then if you did a dirty install of 1.5.1 on top of a previous version.. i suggest a clean install, that is..uninstall k-meleon completely and delete all files(you can make a backup of your profile in there are important settings/bookmarks etc) because maybe with dirty install some old essential files were not rewritten or replaced with the newer ones.
hope this works.. goodluck