  We study the scenarios where a strongly first-order electroweak phase
transition (EWPT) is triggered by a light singlet scalar, which has feeble
interactions to the Higgs. Since the singlet scalar is light and has weak
couplings, it can decay at a macroscopic distance away from the collision
point. Therefore, it can be regarded as a long-lived particles (LLP) in such
scenarios. We perform the searches of the LLPs from the exotic Higgs decays, at
the FASER, MAPP and CMS-Timing detectors of the 14 TeV HL-LHC, to probe the
strongly first-order EWPT. In certain scenarios, we show that the LLP searches
can help to reach the parameter space of the strongly first-order EWPT
remarkably, where the searches for promptly exotic Higgs decays are not valid.
