mrxvt was a nice lightweight multi-tabbed terminal emulator for X. It is still possible to install it through many distributions repositories but we recommend against it. Development of mrxvt stalled in 2008 and the last released version, released that year, is vulnerable to CVE-2017-7483.
Applying the debian patch for rxvt to mrxvt is possible if you really want to use mrxvt for some reason.
There are many much more powerful actively developed terminals with multi-tab support. Plain xterm, which is still actively developed, is a better alternative if you want a minimal terminal.
The homepage at http://materm.sourceforge.net/wiki/pmwiki.php is still alive. With News from.. 2008.