From Old Norse for-, fyr-, fir-, Proto-Germanic *fra-, from Proto-Indo-European *pro- (“before, in front of”).
för-
Often expresses something being done completely or to a natural end point of the action described by the verb, similar to English en- or up. In the case of past participles, the intuition for "förX" is "has had X done to it." Compare for example bränna (“burn”) and förbränd (“incinerated”) ("enburned", "burned up"), krympa (“shrink”) and förkrympt (“stunted in growth”) ("enshrunk", "shrunk up"), multna (“molder”) and förmultna (“decompose”) ("enmolder", "molder up"), blöda (“bleed”) and förblöda (“bleed out”) ("enbleed"), krom (“chrome”) and förkromad (“chrome-plated”) ("enchromed", "chromed up"), and (though with a reversal in meaning from älskad (“loved”)) älska (“love”) and förälskad (“in love”) ("enloved", "loved up"). This is semi-productive (though usually seen as jocular for ad-hoc formations) in modern Swedish, similar to how en- might spontaneously be added to English verbs. See försoffa for an example of this intuition having changed the meaning of a word.