I'm reading Zubrin's book how to live on Mars.
I had hoped for some humor. I can't say I found much.
As Zurbin wrote in A Case for Mars, he seemed to think
there is some need to tie down greenhouse lest they float
away:

"A 340 mb 50-meter diameter dome experiences an upward force of
some 7000 tonnes trying to tear it loose from Martian surface, or
44 tonnes per meter of circumference"
pg 92 of How to Live on Mars

Now, some people might find this extremely funny.
It's absurd, there is no doubt, but it doesn't appeal to my
sense of humor.

Earth's air density is around 1.23 kg per cubic meter.
Helium is about .18 kg per cubic meter
So, on Earth with Helium one gets about 1 kg per cubic meter of lift.

As a general rule one gets less force from buoyancy with lower gravity
and less density of atmosphere. Mars has both these characteristics
as compared to Earth.

A 50 meter sphere has about 65,449.8 cubic meters. If you cut
a 50 meter sphere in half you get a dome 50 meter in diameter
and 25 meters high [which allows you to grow modest size trees in
the middle- assuming you want trees that harvest lumber]
So a 50 meter hemisphere has 32724.9 cubic meters.

On Earth:
The weight of 32,724.9 cubic meters of air is about 40,251.6 kg.
The weight of 32,724.9 cubic meters of helium is about 5890.48 kg
So if you had helium filled greenhouse on Earth which was a 50
meter in diameter hemisphere- 40,251.6 kg minus 5890.48 equals
34361.12 kg buoyancy [34.36 tonnes of force]- if the structure
weighed 35 tonnes it would not float. If you added 340 mb of helium
above the Earth atmospheric pressure [or 4 psi or 1013 + 340 mb of helium],
then buoyancy force is decreased by roughly 2000 kg.

The surface area of 50 meter sphere is 7853.97 sq meter. Half
for hemisphere: 3926.99 sq meters.

"The density of glass varies with each type and ranges from 2000 to 8000 kg/m3"
And:
http://hypertextbook.com/facts/2004/ShayeStorm.shtml

We will use 2500 kg per cubic meter
And have it 2 cm thick. 3926.99 divided by 50 equals 78.54 cubic meter of glass.
Which at 2500 kg per cubic meter weighs 196,349.4 kg or 196 tonnes.
If only 1 cm thick it weighs 98 tonnes. Because of buoyancy the 1 cm thick glass
hemisphere filled with helium would actually weigh on Earth 98 minus the 34.36 tonnes.
This does not need anchoring because of it's buoyancy.
Adding around 4 psi of helium pressure may come close to exceeding the structure strength of 1 cm thick glass [depends upon it's strength] but it will reduce the buoyancy force by around 2 tonnes.

If we replace the glass with a clear plastic having density somewhere around 1000 kg
per cubic meter. And it was 1 cm thick: 98 divide by 2.5 equals 39.27 tonnes. Which is close to being buoyancy neutral [floating] but at 1 atm of helium it still weighs little more: 5 tonnes and about 7 tonnes if it had around 4 psi.

Now let's put the 1 cm thick 50 meter diameter plastic dome with about 2 tonnes of helium on Mars [minus the 1 atm of of helium].
As I said it should have less buoyancy on Mars as compared to Earth.

Mars atmospheric Density = 0.020 kg/m3
32724.9 cubic meters of Mars air weighs 654.5 kg
The helium and dome will weigh a 1/3 less on Mars as compared to Earth.
So, 39.27 tonnes of dome weighs about 13 tonnes.
And 2 tonnes of helium weighs about 666.7 kg
So the helium itself at 340 mb could weigh as much or more than the Mars air.

Temperature will make some difference, but the most lift even if
the helium was at Mars pressure and blazing hot can not
exceed the weight of the Mars air which is 654.5 kg and
since the 1 cm plastic dome weighs 13000 kgs, it's not going
to float.
Reduce the thickness to by a 10th [1 mm walls] and it's
still 1300 kg- twice the most possible lift due to buoyancy.

[Now balloons can be made of thinner lighter plastic and all they need is slightly more temperature and/or pressure than atmosphere. So, I'm not saying balloons don't
work on Mars- but I am saying they don't work better than on Earth- better than
on Earth at 100,000' but not at 5000' or 10,000'.
Zurbin's Mars would be a fanastic place for balloons- balloons which got around 1/4 of the buoyancy as a pipelauncher would get on Earth:)]

So, I would guess that common party balloons doesn't float above the Mars surface- whether inflated with nitrogen or even helium- though weather balloons probably would.
And if you had a big enough balloon, a nitrogen filled balloon could work in the CO2 atmosphere.