  We present Hubble Space Telescope (HST) absolute proper motion (PM)
measurements for 20 globular clusters (GCs) in the Milky Way (MW) halo at
Galactocentric distances $R_{\rm GC} \approx 10-100$ kpc, with median
per-coordinate PM uncertainty 0.06 mas yr$^{-1}$. Young and old halo GCs do not
show systematic differences in their 3D Galactocentric velocities, derived from
combination with existing line-of-sight velocities. We confirm the association
of Arp 2, Pal 12, Terzan 7, and Terzan 8 with the Sagittarius (Sgr) stream.
These clusters and NGC 6101 have tangential velocity $V_{\rm tan} > 290$ km
s$^{-1}$, whereas all other clusters have $V_{\rm tan} < 200$ km s$^{-1}$. NGC
2419, the most distant GC in our sample, is also likely associated with the Sgr
stream, whereas NGC 4147, NGC 5024, and NGC 5053 definitely are not. We use the
distribution of orbital parameters derived using the 3D velocities to separate
halo GCs that either formed within the MW or were accreted. We also assess the
specific formation history of e.g. Pyxis and Terzan 8. We constrain the MW mass
via an estimator that considers the full 6D phase-space information for 16 of
the GCs from $R_{\rm GC} = 10$ to 40 kpc. The velocity dispersion anisotropy
parameter $\beta = 0.609^{+0.130}_{-0.229}$. The enclosed mass $M (<39.5
\rm{kpc}) = 0.61^{+0.18}_{-0.12} \times 10^{12}$ M$_{\odot}$, and the virial
mass $M_\rm{vir} = 2.05^{+0.97}_{-0.79} \times 10^{12}$ M$_{\odot}$, are
consistent with, but on the high side among recent mass estimates in the
literature.
