Electric glass insulators ? Replace or Leave?

Glass was seldom used, more likely they are white porcelain and nearly indestructible. The wire insulation is more of a concern that the insulators. Call a qualified professional; electrician to do an inspection. In most areas, the lines from the power company are their responsibility until the connections are made at your service loop. You are responsible for the service conduit and the wires in there including the meter base. The power company owns the meter.

No. Glass and plastic tend to heat up very quickly and in some cases, plastic may melt if put in extremely hot temperatures. If it does not require boiling something or melting something, I would suggest to use a Styrofoam cup to hold a hot liquid, as it was made for that purpose. Since you can't use Styrofoam, you could use a thermos, which uses a vacuum to insulate the contents. Otherwise, use the glass, it's your best shot.

For safety get an electrician to look at it. Sounds like old wiring that could start arcing.

Glass or porcelain insulators are pretty standard for cable anchors.

Is that what they are?

Always better to modernize.