What FIR boundaries?
My aviation maps for that area do not show the boundaries as depicted in the image p.j.m reposted, only some of the lines (few of them in fact) delineate between Chennai (VOMF), Kuala Lumpur (WMFC), Jakarta, Yangoon and Bangkok (VTBB) FIRs. By leg:
- IGARI-VAMPI crosses in and out of VTBB/WMFC FIRs
- VAMPI-GIVAL follows no boundary and remains inside WMFC FIR
- GIVAL-IGREX remains well inside WMFC.
All based on the (current) SkyVector Hi/Lo alt enroute charts.
What sort of chart was used by the OP?