Config fuses don't affect anything in your code. They must be set manually only when loading the Loader.HEX file the first time.

Code:
@ device pic16F876a, hs_osc ' oscillator selection
@ 4MHZ it should be XT... but the default one should already be XT

Code:
@ device pic16F876a, bod_on ' brown-out reset
If the application run @3V... it may cause some problem... if it's the application i feel it is...

- this bootloader (16F876a_04.hex) works also on a 16LF876A, or?
Should
- the bootloader waits after power up for some serial stuff. the pic doesn't send out anything by himself if he hasn't received something first, right?
Yup!