Bug: Add Remove Programs hangs forever while selecting packages to install

Don't select more than one package at a time. Instead, select one package, then apply; then choose "continue" to choose the next one, and so on. Never all at once.

Could also be that Trisquel doesn't like one of your choices, such as Thunderbird. it's still in the list of programs to add or remove, but choosing it stalls the Add/Remove Programs app. Instead, for Thunderbird, go to "sudo apt-get install Thunderbird", and it'll tell you what to do ... and then it will immediately start downloading icedove, the replacement for Thunderbird, without further intervention from you. DAMHIKT.