Use Wireframesketcher (https://wireframesketcher.com/) for wireframing. it's one of the best out there (cross-platform) and I have been using it on Linux for more than 5 years now. I believe Ubuntu team was using it a while back (not sure whether they still are)
Bought it and wrote a mini review as well (not a pro in writing). It's overall good, not a macbook killer (don't know why everyone loves to pitch against another hardware) nor XPS 13 killer (in terms of ascetics) but an excellent Linux portable laptop.
Bought exclusively from System76 as opposed to DELL in order to support a linux exclusive vendor (DELL has MS as well).
Use Wireframesketcher (https://wireframesketcher.com/) for wireframing. it's one of the best out there (cross-platform) and I have been using it on Linux for more than 5 years now. I believe Ubuntu team was using it a while back (not sure whether they still are)