(abridged) We present the first XMM-Newton observations of the radio-quiet quasar MR2251-178 obtained in 2000 and 2002. The EPIC-pn spectra show a power-law continuum with a slope of 1.6 at high energies absorbed by at least two warm absorbers (WAs) intrinsic to the source. The underlying continuum in the earlier observation shows a soft excess at low X-ray energies which can be modeled as an additional power-law with a slope of 2.9. The high-resolution grating spectrum obtained in 2002 shows emission lines from N VI, O VII, O VIII, Ne IX, and Ne X, as well as absorption lines from the low-ionization ions of O III, O IV, and O V, and other confirmed and suspected weaker absorption lines. We suggest a model for the high-resolution spectrum which consist of two or three WA components. The two-components model has a high-ionization WA with a column density of 10^21.5 - 10^21.8 cm^-2 and a low-ionization absorber with a column density of 10^20.3 cm^-2. In the three-components model we add a lower ionization component that produce the observed iron M-shell absorption lines. We investigate the spectral variations in MR2251-178 over a period of 8.5 years using data from ASCA, BeppoSAX, and XMM-Newton. All X-ray observations can be fitted with the above EPIC-pn model. The 8.5 year history of the source suggests a changing X-ray absorber due to material that enters and disappears from the line-of-sight on timescales of several months. We also present the entire FUSE spectrum of MR2251-178. We detect emission from N III, C III, and O VI and at least 4 absorption systems in C III, H I, and O VI, one at -580 km/s and at least 3 others which are blended together and form a wide trough covering the velocity range of 0 to -500 km/s. The general characteristics of the UV and X-ray absorbers are consistent with an origin in the same gas.
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