if it is the 2501a i know you can downgrade it. i have the 2501a and had very little problem downgrading. i used this guide. You must login or register to view this content.
If you have a 2501 PS3 with a certain manufacturing date you might not be able to downgrade. Because it might have came out with OFW higher than 3.55. If true then you cannot downgrade to 3.55.