See #88

See #88 (comment)

Adding these do not bloat the image too much.

```
The following packages will be downloaded:

    package                    |            build
    ---------------------------|-----------------
    firefox-91.0.2             |       h9c3ff4c_0        72.5 MB  conda-forge
    geckodriver-0.29.1         |       h3146498_0         3.0 MB  conda-forge
    openssl-1.1.1k             |       h7f98852_1         2.1 MB  conda-forge
    selenium-3.141.0           |py37h5e8e339_1002         870 KB  conda-forge
    ------------------------------------------------------------
                                           Total:        78.4 MB
```

See Ouranosinc/raven#402

Previously it was indirectly pulled by RavenPy.  Now RavenPy do not
depend on it anymore.

See CSHS-CWRA/RavenPy#134 and
CSHS-CWRA/RavenPy#133.

…or bokeh png export

The `firefox` install by `conda` was missing all the X-libraries so did not work:

```
! firefox --version

XPCOMGlueLoad error for file /opt/conda/envs/birdy/bin/FirefoxApp/libmozgtk.so:
libgtk-3.so.0: cannot open shared object file: No such file or directory
Couldn't load XPCOM.
```

See #88 (comment)

Error when only `firefox-esr` is installed by `apt-get`:
```
! firefox --version

XPCOMGlueLoad error for file
/opt/conda/envs/birdy/bin/FirefoxApp/libxul.so:
libXt.so.6: cannot open shared object file: No such file or directory
Couldn't load XPCOM.
```

See #88 (comment)